High‐mobility suspended MoS 2 devices with tunable threshold voltage

Threshold voltage variation is a problem in field effect transistors (FETs) fabricated from arrays of aligned semiconducting carbon nanotubes. Moreover, the subthreshold swing of FETs fabricated from arrays of nanotubes is often several times larger than the subthreshold swing of FETs fabricated from a single nanotube or nanotube bundle. Here, we experimentally quantify the impact of nanotube bundling and array non-uniformity on threshold voltage variation and subthreshold swing degradation in arrays of aligned semiconducting nanotubes deposited by floating evaporative self-assembly. The height (and, therefore, degree of bundling) and array morphology of more than one hundred back-gated FETs (with the nanotube channel exposed) are mapped via atomic force microscopy. Both threshold voltage and subthreshold swing significantly increase as the degree of bundling increases—showing a clear link between the physical characteristics of an array and electrical properties. Modeling shows that much of the subthreshold swing degradation in arrays can be attributed to the bundling–threshold voltage link, in which polydispersity in bundle size within an array causes a bundle-by-bundle shift in threshold voltage and a gradual turn-on of the FET. Increased off-current also plays a role in increasing the sub-threshold swing. These findings highlight the importance of synthesizing and assembling arrays of aligned nanotubes that are highly ordered, with uniform and repeatable morphology.

An ultra-thin natural-source-based cellulose derivate is used in a bilayer gate dielectric for organic complementary inverters. Extraordinarily high small-signal gains up to 1600, a very sharp transition region (as low as 40 mV), a full rail-to-rail swing, a balanced noise margin of 92.5%, and “long-time” stability are shown, revealing that these inverters are highly suitable for complex circuit designs. For more than a decade now the research in organic electronics was driven by unique consumer markets demanding for electronic circuits on flexible substrates over large areas and ideally at low cost. Meanwhile, versatile macroelectronic applications with attractive form factors such as flexible displays,1-4 large-area sensor arrays,5-11 e-paper,12, 13 low-cost radio-frequency identification tags,14-18 and bendable microprocessors19 have been realized using organic thin-film transistors (OTFTs) as the essential element. In the majority of these applications OTFTs are integrated either in the form of arrays or as digital logic. The fundamental building block of digital logic is the inverter, a circuit that inverts an input signal. Much work has been devoted in the last five years to design unipolar inverters using p-type-only OTFTs.18-23 Unfortunately, unipolar logic based on single gate OTFTs always suffers from high power consumption and low noise margin coupled with high noise margin variability and therefore impedes the fabrication of complex circuits having hundreds or more logic gates.19, 22, 23 In this context, it stands beyond doubt that the integration of both n-type and p-type transistors in complementary organic-based circuits12 is very advantageous due to a lower power dissipation, higher noise margin, better operational stability, and easier design of the circuit.24-27 To date, several major obstacles complicated the development of organic complementary circuits, which are as follows: i) the lack of n-channel OTFTs with a performance comparable to the p-channel OTFTs, ii) the challenge of device stability (difficulty in fabricating OTFTs with hysteresis-free characteristics), and iii) the difficulty in fabricating sufficiently thin gate dielectrics allowing for low supply voltages. Although many recently reported complementary organic inverters are impressive with respect to their low operation voltages (≤5 V),27-36 high static gains (>100 V/V),25, 28, 30, 32 and large noise margins (>80%),25, 35, 37, 38 only few of those can combine low-voltage operation with high gain and sufficient immunity against electrical noise.39 Moreover, there have been few investigations of the influence of hysteresis and threshold voltage shifts on the inverter characteristics, which typically gives rise to electrical instabilities and performance limitations of those devices.32, 39 The inverter's noise margin is the most critical parameter, since it directly determines the maximum complexity of circuits.22 Particularly, interesting applications of organic electronic circuits are found in analog electronics that allow for the read-out and processing of signals from printed large-area sensors and form an interface to silicon electronics. Examples are analog-to-digital (A/D) converters, operational amplifiers, and comparators, all including several tens of transistors, thus requiring both high gain and a large noise margin to be functional as well as sufficient processability. It is interesting to note that the majority of organic complementary inverters already mentioned24-34, 36-40 relied on pentacene as the p-type semiconductor (14 incidences), fluorinated copper phthalocyanine (6 incidences), or N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (5 incidences) as the n-type material. On the contrary, the variation in gate dielectric materials that are used is much higher with an emphasis on hybrid dielectric systems. Hybrid systems allow accounting for both a high dielectric breakdown voltage and an engineered interface to the semiconductor. Moreover, the implementation of biodegradable and biocompatible materials represents a new niche in the organic electronics research, aimed to address the issues of cost and toxicity to humans and environment posed by nowadays electronics, a topic that was recently reviewed elsewhere.41-43 Accordingly, we have demonstrated the usage of a novel hybrid and biodegradable high-k gate dielectric material system based on a cellulose derivative for low-voltage complementary organic circuits with exceptional high noise margins.35 Previous work done in our laboratories demonstrated the fabrication feasibility of complex circuits, where the electrode patterning was performed by a self-aligned photolithography technique.44, 45 This process can easily be transferred to an anodized aluminum (Al2O3) + trimethylsilyl cellulose (TMSC) based bilayer system; the respective results will be published soon. To follow on this preliminary work we have developed an extraordinarily well-performing complementary inverter technology based on a hybrid dielectric composed of a bilayer of alumina (Al2O3) and trimethylsilyl cellulose (TMSC) and pentacene or C60 for the p-channel and the n-channel organic semiconductor, respectively. It outperforms any inverter reported so far with respect to the combination of excellent key parameters, such as record DC gain of above 500 V/V, low operation voltage of only 4 V, and large noise margin up to 92.5%. In order to clarify the origin for this high performance, special attention is given to the investigation of the semiconductor–dielectric interface, the semiconductor morphology, the nature of traps, the influence of mobility levels on the inverter performance and its long-term stability. This article is not meant to report new materials and systems, but rather find explanations and present a thorough analysis of the extraordinary performance of the cellulose derivative (TMSC) dielectric and the TMSC to semiconductor interface in organic field-effect transistors and simple inverter circuits. All OTFTs presented in this work have a bottom gate, top contact device architecture, as illustrated in Figure 1a. The transistor fabrication process is described in detail in the Experimental Section. The semiconducting layer is pentacene for p-type and C60 (fullerene) for n-type conduction. For the gate dielectric, an inorganic (Al2O3)–organic (TMSC) bilayer is chosen with 28 and 30 nm layer thickness, respectively. The dielectric properties of both single-layer TMSC films and bilayer Al2O3 + TMSC films, were investigated by current–voltage (I(V)), capacitance–frequency (C(f)), and capacitance–voltage (C(V)) measurements in appropriate capacitor structures with 72 nm insulator layer thickness of pure TMSC and 28 + 30 nm of bilayer film of Al2O3 + TMSC, respectively. In Figure 1b, the I(V) curves of a representative capacitor formed by a single layer cellulose dielectric is shown, whereas in Figure S1a (in the Supporting Information) the curve with bilayer dielectric is displayed. A comparison of I(V) curves of a single layer and bilayer dielectric is displayed in Figure 1c. At an electric field of 3.0 MV cm−1 (equivalent to ≈22 V) a current density on the order of 10−6 A cm−2 is observed for the single-layer TMSC. A breakdown field of 4.5 ± 0.3 MV cm−1 is extracted from eight appropriate capacitor structures. One representative C(V) measurement is displayed in Figure 1d. Its capacitance of about 29 nF cm−2 corresponds to a dielectric constant of εR = 2.4 ± 0.2 at 1 kHz with an excellent stability at frequencies down to 0.5 mHz (virtually flat dielectric constant and low dielectric losses which are indicative of a high purity and negligible amount of mobile ionic species) as illustrated in Figure 1e.46 In comparison, capacitors based on the bilayer dielectric have three orders of magnitude lower leakage currents (≈10−9 A cm−2) at an electric field of 3.0 MV cm−1 corresponding to ≈17.5 V and an effective capacitance of 53 nF cm−2 at 1 kHz (see Figure S1a,b, Supporting Information). As can be derived from Figure S1c (in the Supporting Information) the permittivity of the bilayer dielectric also shows excellent stability over the tested frequency range (1 mHz to 10 kHz). The measured capacitance value of the dielectric is in good agreement with calculations by assuming a permittivity of 9 and a thickness of 28 nm for the Al2O3 layer as well as a measured permittivity of 2.4 ± 0.2 (Figure 1e) and 30 nm thickness for the TMSC layer (see the Experimental Section). In any OTFT with bottom gate architecture the surface properties of the gate dielectric (surface energy, surface roughness) decisively affect the arrangement and growth of small molecule semiconductors. The surface energy of spin-coated TMSC thin films was determined by contact angle measurements. With a water contact angle of 98.5°, TMSC can be regarded as a hydrophobic, low energy surface material with a total surface energy of γ = 24.2 ± 0.6 mN m−1, separable in a small polar component γP = 1.8 ± 0.5 mN m−1 and a dispersive component γD = 22.4 ± 0.1 mN m−1. The average surface roughness, indicated by the root mean square (rms)–roughness values, was found to be only 0.77 ± 0.1 nm by atomic force microscopy (AFM) measurements. On this smooth and low energy TMSC surface, a small-grain growth mode with average grain sizes between 150 and 300 nm for pentacene (shown in Figure 1a) and 50 nm for C60 was observed by AFM measurements. In Figure 2, typical electrical characteristics of OTFTs with either C60 or pentacene as the semiconducting layer are plotted. Both p- and n-type transistors are based on the inorganic–organic bilayer gate dielectric composed of Al2O3 and TMSC. The output characteristics ID(VDS) are displayed in Figure 2a,b, whereas the transfer characteristics ID(VGS) are shown in Figure 2c,d. Both transistor types display very small hysteresis between forward and reverse voltage sweeps. The output characteristics show clear saturation, but for the pentacene-based OTFT a non-linear behavior of the drain current at very low drain voltages is observable (Figure 2b). Such an S-shape of the output curve has been already observed and reported for many different device configurations and has sometimes been attributed to a lateral field dependent contact resistance.47-55 In Table 1, the critical device parameters Von (onset voltage), Vthr (threshold voltage), S (subthreshold swing), μlin (charge carrier mobility in the linear regime), and μsat (charge carrier mobility in the saturation regime) are compiled and averaged over eight devices for each transistor type. On average, C60-based OTFTs show a charge carrier mobility of 0.7 cm2 V−1 s−1 in the saturation regime, a threshold voltage of 1.3 V, and a small subthreshold swing of 305 mV dec−1. Pentacene-based OTFTs have on average a mean threshold voltage of −4.7 V, a very small subthreshold swing of 225 mV dec−1, and a field-effect mobility of 0.22 cm2 V−1 s−1 in the saturation regime. The gate leakage currents are very low and in the range of 10−10 A. The balanced performance of our p- and n-type OTFTs, as well as the fact that both transistor types strictly behave as normally off transistors at zero applied gate voltage is a good base for integrating them in complementary organic inverters. A schematic image of such a complementary inverter circuit is displayed in Figure 3a together with the voltage transfer characteristics (VTCs) of a typical device and the static gain measured for operation voltages of VDD = 4, 5, 6, and 7 V. The inverter exhibits extraordinarily high small-signal gains; for all operation voltages, gains above 500 V/V are measured with a maximum value of about 1000 V/V for VDD = 7 V. The best performing device even has a maximum gain value of 1600 V/V. Moreover, a very sharp transition regime (ΔVin ≤ 40 mV) and a full rail-to-rail swing are observed. The extracted inverter parameters are compiled in Table 2. The switching threshold voltages VM are found to be very close to the ideal value of VDD/2 at all operation voltages with a nearly perfect fit (e.g., VM = 2.04 V at VDD = 4 V). In Figure 3b, the mismatches of the threshold voltages to the ideal values (ΔVM) are plotted as a function of VDD revealing a linear increase of ΔVM with the supply voltage. This means that the asymmetry of the VTCs increase with increasing the operation voltages. Additionally, the noise margins (NM) are plotted in Figure 3b as a function of VDD, illustrating the linear decrease of the NM with increasing operation voltage. Such a behavior reveals the direct correlation between ΔVM and NM and is an indication of the deviation of the VTCs from perfect symmetry. The noise margin is an important figure of merit of an inverter stage and measures the immunity against inevitable transistor parameter variations and electrical noise.56 There are several ways to determine the noise margin of an inverter. One convenient way is called the “maximum equal criterion” (MEC), where the NM is found by the maximum square that fits between inverter transition curve and the mirrored inverter curve as shown in Figure 3c.22, 35 The NM values for all the supplied voltages are listed in Table 2. The noise margin is mainly limited by the steepness of the transition ΔVin and the position of the switching threshold voltage VM (VM = Vin = Vout) as shown in Figure 3c. High and balanced noise margins are found when the ideal condition VM = VDD/2 is met, thus when VM = Vin =VDD/2 = Vout. The inverter in Figure 3c has a NM of 1.79, which is 90% of the maximum theoretical value for a supply voltage of 4 V, determined by the “maximum equal criterion.” Additionally, the “negative slope criteria” reveals a balanced NM of 1.85 V (92.5%) for a supply voltage of 4 V. The OTFTs with pentacene and C60 semiconductor layers (as used in the complementary inverters) were also analyzed with regard to their long-term stability. The dependence of their onset voltage and charge carrier mobility on the time period after preparation is plotted in Figure 4 based on measurements of at least five devices for each transistor type. The error bars display the standard deviations. p- and n-type devices exhibit an onset voltage shift of +1.3 and +0.95 V, respectively, over a period of 45 days. The main part of the shift occurred either after the first or after the second measurement and then the onset voltage shift tended to saturate. It seems that both pentacene- and fullerene-based devices stabilized their electrical properties after few days. Between day 8 and day 45 no significant change in the transfer characteristics was observed. Concerning the subthreshold swing, no significant change was observed over 45 days (S = 215 mV dec−1 (0 days), 210 mV dec−1 (8 days), and 225 mV dec−1 (45 days)) in the pentacene OTFTs, indicating that the interface trap density is stable. On the contrary, after 8 days an increase of the swing was observed for the C60 devices from 305 mV dec−1 (0 days), to 410 mV dec−1 (8 days), to 430 mV dec−1 (45 days). The mean value of the charge carrier mobility of the pentacene OTFTs showed no significant variation over 45 days: 0.22 cm2 V−1 s−1(fresh sample), 0.19 cm2 V−1 s−1 (after 8 days), 0.21 cm2 V−1 s−1 (after 45 days). In contrast, a strong reduction of about one order of magnitude (from 0.7 to 0.06 cm2 V−1 s−1 within the first 8 days and to 0.05 cm2 V−1 s−1 after 45 days) was observed for the fullerene transistors. In order to test the reliability of our OTFT devices we have also performed negative and positive bias stress tests. The bias stress was done 1 day after fabrication, on devices which had been previously measured only once. A reversible negative onset voltage shift of about 400 mV was observed under negative bias of −7.5 V over 21 hours stress (see Figure 5a,b) The main part of the negative voltage shift occurred during the first 2 hours (300 mV) whereas shift saturation was reached after 6 hours. The positive bias stress resulted in a significantly smaller positive onset voltage shift (ca. +100 mV over 4 hours stressing the device). Devices that were stored for seven months in the glovebox showed no bias stress effect at all (i.e., shift of the onset voltage) when their transistor characteristics were measured at regular intervals during the storage period. In order to investigate the “long-time” behavior of complementary inverters, electrical measurements were done after 1, 4, 8, and 45 days of the respective device preparation. Inverters were measured 1 day after device stabilization (see Figure 4a,b). The “long-time” behavior of complementary inverters is shown in Figure 4c,d where the VTC and gain curves are plotted as a function of time. For the lowest VDD of only 4 V, a small shift of the switching threshold voltage was measured: starting from 2.04 V after 1 day to an identical value of 2.17 V after 8 and 45 days, respectively, with a related exceptional high noise margin of 1.79 V (90% of the theoretical maximum) after 1 day and 1.74 V (87%) after 8 and 45 days. It is interesting to note that, after 8 days of storage time, the hysteresis in inverter characteristics was virtually negligible (<100 mV, see Figure 4d) and moreover, repeated measurements did not change the inverter transition, although the gain was somewhat reduced (from 503 (0 days) to 320 (8 days) and 195 (45 days)). The operational stability of the VTC was shown over a period of at least 45 days (Figure 4c for VDD = 4 V). Furthermore, the remarkable long-term stability of these TMSC-based complementary inverters was demonstrated by 16 consecutively recorded inverter curves that are virtually superimposed over one another (Figure 5c). The current–voltage characteristics show that the Al2O3 + TMSC bilayer capacitors have substantially lower leakage currents than the single TMSC layer devices. Due to the roughness of the aluminum bottom electrode with spikes up to 10 nm, the anodization process is a convenient method to reduce the leakage currents for at least three orders of magnitude at an electric field of 3 MV cm−1 as displayed in Figure 1d and make the dielectric layer more robust.57 Another advantage of the anodization step is the substitution of the hydroxide of the native oxide layer. Finally, the subsequent short oxygen plasma treatment (see the Experimental Section) is sufficiently removing all residuals left after the anodization process. This procedure allows for the generation of a pure and trap-reduced dielectric interface to the organic semiconductors, by measurements at low revealing a virtually flat permittivity in the mHz to kHz frequency range and low dielectric losses which both are indicative of a high purity and negligible amount of mobile ionic The generation of this excellent interface is in virtually hysteresis-free characteristics of all the One important that the performance of OTFTs, and in the charge carrier is the of the As shown by several the of the surface energy of the to that of the is one important the growth For a low surface energy material with a smooth surface TMSC, a growth mode is and with a rather high density and small grain sizes are formed (see Figure The high density of grain a significant decrease of the charge carrier since grain as charge On the a better between pentacene on low surface energy materials TMSC with and better reduce the of trap thus the mobility of charge in the In there is an between grain related and interface with regard to their influence on charge carrier The low subthreshold swing observed in all our pentacene-based OTFTs is indicative of a low interface trap density which the influence of the large grain trap density on the charge carrier Accordingly, a mobility 0.2 cm2 V−1 s−1 is observed in the p-type devices. In C60 films, the influence of on the charge carrier mobility is by far not as significant as in pentacene films, which is attributed to the of the fullerene In the pentacene OTFTs (see Figure a non-linear current increase in the linear regime of the ID(VDS) output characteristics is at higher gate have been in to such a which can be observed in bottom gate The explanations i) the of the charge carrier mobility for semiconductor ii) currents that the of the mobile by an effective under the drain and iii) large at the electrode from an energy between the work of contact material and semiconductor films, a gate or large contact are found in our of these it has been shown that a mobility in combination with a small contact in a non-linear drain voltage dependence of the current due to The from a of from trap and a dependence of on the drain A respective of the pentacene output curve in Figure the dependence in the regime 2 V, thus indicating that a mobility has to be The of any hysteresis in the I(V) curves and the low and value of the subthreshold swing a low density of interface in the pentacene are not for the observed and the of the as by the of the pentacene layer gives rise to which to a field dependence of the charge carrier This is all the more given that the pentacene grain sizes are 300 nm at a large density of grain As by Figure 4 the characteristics of both pentacene- and C60-based OTFTs show a positive onset voltage shifts storage which after a of measurements. The virtually subthreshold swing in pentacene as well as the negligible hysteresis as the origin of the On the contrary, it seems that trap are for the positive onset voltage shift in are either over time or during the first measurements by the applied positive gate be easily by the gate bias and therefore are not to the swing, but the small onset voltage in pentacene-based It has been reported by many that the of the of trap in pentacene trap also be due to such as at the a at the or a at the fabrication and transfer from the to the measurement stage the semiconductor layer and rise to such For oxygen can even the devices are to but transferred directly and measured in a glovebox with reduced oxygen and water have also observed that the of oxygen with pentacene is in the that these are under device operation with the of oxygen in the grain observed that the storage of devices in and their subsequent measurement in did not a change in their I(V) also observed that the storage time to the first measurement has influence on the device (i.e., on the onset voltage in this of and the first measurement the of to the of oxygen or with the which in a second are by Moreover, also are are not as in the as the (see and This an for the positive shift of the onset voltage between first and second measurement (Figure and also for the saturation of the onset voltage shift in In order to clarify which of are present in our we performed measurements on pentacene films (in and after storage in for (see Figure Supporting Information). storage under a small increase of the at energy is observed that can be attributed to the to and The analysis reveals an increase of the from 0.2 to At the position no change is that trap in our devices for bias stress and the positive onset voltage can mainly be attributed to It be that the of any mobility in pentacene OTFTs against an of pentacene both are to oxygen For the C60 transistors, one a strong decrease of the charge carrier mobility during 8 days of storage which is by all over the device grain This is a effect of The decrease in charge carrier mobility and the at the interface that the subthreshold swing is in a positive onset voltage shift of the C60 transistors. With respect to the complementary inverters, noise margin values beyond from the maximum theoretical value for all supply voltages are and the high operational of the OTFT devices. At a very low supply voltage of 4 V, our bilayer organic complementary inverters have a static gain of more than 500 V/V with a noise margin of 92.5%, which is already reported devices with architecture (see the of inverter performance given in 38 and also gain values above 1000 V/V values up to 1600 were measured for our devices. This is all the most as the charge carrier mobility values of the transistors are not thus the that the of the mobility values between p- and n-channel with the virtually hysteresis are the factors for high gain and high noise margin Accordingly, a mobility 1 be the the is not high ≤ it can be by and p-channel transistors with different In our the mobility is The static of a complementary inverter is determined by technology parameters, the parameters of p- and n-type transistors. A good of these parameters a good saturation of both output can to a significant increase of the gain A is shown in the Supporting Furthermore, high gain values are also determined by the interface An insulator with an low trap density (as this is the for is in having I(V) curves with virtually zero Accordingly, it is to measurements with a very high voltage small voltage thus one to the high gain value of the complementary inverter it by hysteresis The noise margin with increasing VDD due to their strong this is also for the of the VTCs by see Figure A for this behavior can be found in the bias stress that was observed for both types of transistors. The negative shift of Von under negative gate and the positive

Modeling of bias-induced changes of organic field-effect transistor characteristics

A multi-gate n-type In 0.53 Ga 0.47 As metal oxide semiconductor field effect transistor is fabricated using the gate first self-aligned method and air-bridge technology with an 8 nm thick Al 2 O 3 oxide layer. By scaling the gate length down to 200 nm, the effect of two different source/drain doping concentrations on device parameters such as threshold voltage, I on / I off ratio and subthreshold swing were investigated at room temperature. Increasing the value of the source/drain doping concentration revealed an enhancement in all investigated parameters. Sheet resistance and contact resistance values were improved as well. The negative shift in threshold voltage for shorter gate lengths was observed for both source/drain concentrations; however, the shift in threshold voltage was less (~0.4 V) for the higher source/drain doping concentration.

Oxide-based thin-film transistors (TFTs) with a lateral in-plane electrode are self-assembled on paper substrates, and the electrical modulation effect of the in-plane electrode is investigated. A SiO2-based solid-electrolyte film with high specific capacitance is used as the gate dielectric, and the operation voltage is reduced to less than 2.0 V. The threshold voltage (Vth) of such paper TFTs is tuned from -0.98 to 0.94 V by different voltage biases on the in-plane electrode. The threshold voltage shift (ΔVth) can be described by ΔVth = -(CG2/CG1)VG2, where CG2 and CG1 are the in-plane electrode and bottom-gate specific capacitance values. High electrical performance with a current on/off ratio of 6 ×105 ~ 106, a subthreshold swing of 0.14 ~ 0.19 V/dec, and a mobility of 8.64 ~ 9.45 cm2/V·s is obtained at different in-plane electrode voltage biases. Such low-voltage paper TFTs are promising for low-cost and portable electronics.

We examined the possibility of measuring dissolved oxygen by using a potentiometric solid-state semiconductor sensor. Thin films of tin (IV) oxide (SnO2) are widely used in oxygen gas sensors. However, their ability to detect dissolved oxygen (DO) in solutions is still unknown. In this paper, we present a method for investigating the dissolved oxygen-sensing properties of SnO2 thin films in solutions by fabricating a SnO2-gate field-effect transistor (FET). A similarly structured hydrogen ion-sensitive silicon nitride (Si3N4)-gate FET was fabricated using the same method. The transfer characteristics and sensitivities were experimentally obtained and compared. The transfer characteristics of the FET show a shift in threshold voltage in response to a decrease in DO concentration. The SnO2-gate FET exhibited a sensitivity of 4 mV/ppm, whereas the Si3N4-gate FET showed no response to DO. Although the SnO2-gate FET responds to pH changes in the solution, this sensitivity issue can be eliminated by using a Si3N4-gate FET, which is capable of selectively sensing hydrogen ions without DO sensitivity. The experimental results indicate the promising properties of SnO2 thin films for multimodal sensing applications.

Lanthanum aluminate (LaAlO3) is considered as a potential candidate for high-K dielectric applications. LaAlO3 has high dielectric constant (13-27), large energy band gap (over 5eV), high thermal stability (up to 850°C), low leakage current density, large electron band offset. The structure of Al/LaAlO3/Si capacitors and transistors were fabricated successfully. The oxide films were deposited by rf-sputtering. The leakage current density was 4.36×10-3A/cm2 when the applied voltage was -1V. The measured dielectric constant was 13.4. The dominant electrical conduction mechanism of LaAlO3 thin film was Schottky emission mechanism at 450K. The electron barrier-height between Al/LaAlO3 interface and electron effective mass in the LaAlO3 film were about 0.94 eV and 0.07 m0, respectively. Secondary ion mass spectrometry (SIMS) and X-ray diffraction (XRD) were also used to examine the material properties of LaAlO3. The electrical characteristics of N-channel metal-oxide-semiconductor field effect transistors were also measured. The interface trapped charge density, effective electron mobility and sub-threshold swing were 2.27x1012cm-2-eV, 210cm2/V-s and 82.8 mV/dec., respectively. The degradation mechanisms of effective electron channel mobility and the threshold voltage shift in LaAlO3-gated n-MOSFETs were studied by analyzing experimental data at various temperatures from 11 K to 450 K. The mechanisms that influence the electron mobility include coulomb scattering, phonon scattering, surface roughness scattering.

A multi-gate n-type In0.53Ga0.47As MOSFET is fabricated using gate-first self-aligned method and air-bridge technology. The devices with different gate lengths were fabricated with the Al2O3 oxide layer with the thickness of 8 nm. In this letter, impact of gate length variation on device parameter such as threshold voltage, high and low voltage transconductance, subthreshold swing and off current are investigated at room temperature. Scaling the gate length revealed good enhancement in all investigated parameters but the negative shift in threshold voltage was observed for shorter gate lengths. The high drain current of 1.13 A/mm and maximum extrinsic transconductance of 678 mS/mm with the field effect mobility of 364 cm2/Vs are achieved for the gate length and width of 0.2 µm and 30µm, respectively. The source/drain overlap length for the device is approximately extracted about 51 nm with the leakage current in order of 10−8 A. The results of RF measurement for cut-off and maximum oscillation frequency for devices with different gate lengths are compared.

Simulation based investigation of Recessed Dual-Gate MISHEMT on sapphire substrate has been presented in this work using ATLAS simulation software. Various DC performance parameters such as: threshold voltage shift, drain current and transconductance has been compared for different gate combinations. Threshold voltage variation is observed to be nearly same if gate1 is recessed only or both gates are recessed. Positive shift in threshold voltage has been observed as the depth of gate recess is increased from 0 nm to 8 nm. Enhancement in negative junction depth from 10 nm to 18 nm results in the shift in threshold voltage towards positive i.e. −3.5 V to −2.45 V. This improvement in threshold voltage is due to the reduction in barrier thickness with increase in negative junction depth. DC performance has also been evaluated for the device with high-k gate dielectric such as HfO2 and TiO2 for non-recessed and recessed device. The parameters like gate oxide and work function variation results in the shift of threshold voltage from −3.5 V to 0 V in recessed Dual-Gate MISHEMT.

Threshold voltage shifts in metal-oxide-semiconductor field-effect transistors with HfySiOx gate layers and SiO2/HfySiOx gate stacks have been simulated, taking into account the generation of Si trivalent dangling bonds at the Si/dielectric interface, resulting from the injection of electrons through the structure. While the tunneling current flowing through devices with HfySiOx single layers is predicted to be lower compared to devices with SiO2/HfySiOx gate stacks (with equivalent electrical thickness), it is found that the transistor lifetime, based on threshold voltage shifts, is improved in SiO2/HfySiOx gate stacks. This finding is attributed to the beneficial presence of the SiO2 interfacial layer, which allows the relaxation of strain at the Si/dielectric interface.

The dosage of 5-fluorouracil (5-FU), which is a widely used for cancer medication, is determined based on body surface area, although efficacy largely depends on the liver function of the individual, resulting that only 21% patients are given an optimal dose of 5-FU in these years. The measurement of the 5-FU concentration in the blood enables us to adjust individual dose adjustment. Conventional methods for 5-FU detection such as an enzyme-linked immunosorbent assay (ELISA) and liquid chromatography are not very suitable for clinical applications because they need time-consuming procedure with expensive equipments. Detection of 5-FU using a field effect transistor (FET) biosensor, which enables rapid and simple measurement, is expected to solve such problems. However, FET biosensor, which detects changes in its surface density due to the adsorption of charged molecules, was unable to detect uncharged 5-FU. In this study, a method for the FET biosensing to detect 5-FU exploiting sequential adsorption of 5-FU modified bovine serum albumin (BSA/5-FU) was proposed. By using this method, FET responses caused by the adsorption of negatively charged BSA/5-FU depending on the 5-FU concentration were detected. The SiO2 surface of the FET gate insulator was exposed to O2 plasma to introduce hydroxyl groups. After the exposure, the surface was exposed to 3-aminopropyltriethoixysilane (APTES), followed by the modification of the cross-linker, glutaraldehyde (GA). A single chain variable fragments (ScFv) and antigen binding fragments (Fab) were allowed to react with each activated GA-modified FET. After the immobilization, the residual aldehyde-groups were treated by ethanolamine to suppress the non-specific adsorption. V g-I d characteristics were measured before and after dripping of both 5-FU and BSA/5-FU on the ScFv- and Fab-immobilized FET biosensors. Finally, threshold voltage shifts (∆V g) caused by the adsorption of BSA/5-FU were obtained. To compare the capture capability of ScFv and Fab, the electrical responses of the FET biosensors functionalized with the two receptors due to the adsorption of BSA/5-FU were measured. The responses of ScFv- and Fab-immobilized FET biosensor caused by dripping of 25 μg/mL BSA/5-FU were +25 mV and +40 mV, respectively. The difference between ΔV g values for these two FET sensors using ScFv or Fab can be ascribed to the difference of affinity [1]. To verify the specificity of Fab-immobilized FET biosensor, ∆V g was measured when human serum albumin (HSA) was dropped on the FET biosensor, and the ∆V g was hardly observed. Additionally, the atomic force microscopic (AFM) images on the FET gate surface shows that the size of observed particles matches the size of BSA/5-FU, while the surface morphology and roughness are not significantly changed. These results indicated that Fab-immobilized surface specifically captured BSA/5-FU. To investigate the quantitative detectability of the Fab-immobilized FET biosensor, we measured the FET responses corresponding to the amount of adsorbed BSA/5-FU, which was related with the concentrations of 5-FU. As a result, the magnitude of ∆V g by dripping of 1000 ng/mL 5-FU and 25 μg/mL BSA/5-FU was reduced to +12 mV compared with the response of 25 µg/mL BSA/5-FU (Figure 1). These results can be attributed to that the adsorbed 5-FU inhibited the adsorption of BSA/5-FU to the Fab-immobilized surface. Therefore, we conclude that the detection of 5-FU using the FET biosensors by applying the charged BSA/5-FU is a promising simple method for monitoring the concentration of 5-FU.Reference:[1] Y. Reiter, et al., J. Biol. Chem., 269, 28, 18327-18331 (1994).Figure 1 V g-I d characteristics of Fab-immobilized FET biosensor before and after dripping of (a) 0 ng/mL 5-FU and 25 µg/mL BSA/5-FU or (b) 1000 ng/mL 5-FU and 25 µg/mL BSA/5-FU. Figure 1

The authors present a new low-temperature nanowire fabrication process that allows high-aspect ratio nanowires to be readily integrated with microelectronic devices for sensor applications. This process relies on a new method of forming a close-packed array of self-assembled high-aspect-ratio nanopores in an anodized aluminum oxide (AAO) template in a thin (2.5 μm) aluminum film deposited on a silicon substrate. This technique is in sharp contrast to the traditional free-standing thick film methods, and the use of an integrated thin aluminum film greatly enhances the utility of such methods. The authors have demonstrated the method by integrating ZnO nanowires onto the metal gate of a metal-oxide-semiconductor (MOS) transistor to form an integrated chemical field-effect transistor (ChemFET) sensor structure. The novel thin film AAO process uses a novel multistage aluminum anodization, alumina barrier layer removal, ZnO atomic layer deposition (ALD), and pH controlled wet release etching. This new process selectively forms the ZnO nanowires on the aluminum gate of the transistor while maintaining the remainder of the aluminum film intact for other integrated device components and interconnects. This self-assembled high-density AAO template was selectively formed in an ultrasmooth 2.5 μm thick aluminum layer deposited through e-beam evaporation without the electropolishing required in AAO template formation in traditional 100 μm thick free standing films. The resulting nanopore AAO template consists of nanopores of 90 nm in diameter and 1 μm in height at an aerial density of 1.3 × 1010 nanopores/cm2. This thin film AAO template was then filled with ZnO using ALD at 200 °C, forming polycrystalline ZnO nanowires inside the pores. The alumina template was then removed with a buffered NaOH solution, leaving free standing ZnO nanowires of 1 μm height and 90 nm diameter, offering an increase in 38× the surface area over a standard flat ZnO film for sensing applications. The aluminum film remains intact (unanodized) in nonselected regions of the device as well as underlying the ZnO nanowires, acting as the gate of the MOS transistor. The ZnO nanowires were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and transmission electron microscopy to verify stoichiometry and crystal structure. Additionally, the response of a ZnO nanowire ChemFET was measured using ammonia as a target gas. I-V characterization and transient response to ammonia in the range of 25–200 ppm were examined. The ammonia response to the threshold limit value concentration of ammonia (25 ppm) shows a 56 mV shift in threshold voltage, an overall sensitivity of 14%, an 8 min response time, and a 27 min recovery period. The ZnO nanowire fabrication sequence that the authors present is accomplished at low-temperature (&amp;lt;200 °C) and can be accomplished selectively, making it readily amenable to integration with standard metal-oxide-semiconductor field-effect transistor processing as well as other microelectronic sensors such as surface acoustic wave devices. This new process has initially been demonstrated using ZnO, but is also adaptable to a variety of nanowire materials using appropriate deposition methods as well as selective nanowire release methods. This allows the potential to conveniently fabricate a variety of high-aspect ratio nanowire based microelectronic sensors for a range of applications.

The effects of N2 plasma interface treatment and atmospheric pressure chemical vapor deposition (APCVD) silicon dioxide (SiO2) gate insulator were investigated for the performance and the stability of hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs). To improve an a-Si:H/SiO2 interface, the N2 plasma treatment was carried out on the SiO2 surface. As a result, both the threshold voltage and subthreshold voltage swing were decreased remarkably. The high performance of a-Si:H TFTs was achieved with a field-effect mobility of 1.27 cm2/V s, threshold voltage of 3.5 V, and subthreshold voltage swing of 0.4 V/decade. For both positive and negative gate bias stresses, the threshold voltage was shifted in the positive direction and was dominated by the defect creation near the a-Si:H/SiO2 interface. There was little charge trapping into the APCVD SiO2 gate insulator. For the positive gate bias and time stress, the threshold voltage shifts were smaller than those of the conventional a-Si:H TFTs with the a-Si:H/SiNX interface.

Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2020Multi-Stimuli-Responsive Field-Effect Transistor with Conjugated Polymer Entailing Spiropyran in the Side Chains Jing Ma†, Jianwu Tian†, Zitong Liu, Dandan Shi, Xisha Zhang, Guanxin Zhang and Deqing Zhang Jing Ma† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) University of Chinese Academy of Sciences, Beijing 100049, (China) , Jianwu Tian† Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) University of Chinese Academy of Sciences, Beijing 100049, (China) , Zitong Liu *Corresponding author: E-mail Address: [email protected]; E-mail Address: [email protected]; Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) , Dandan Shi Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) University of Chinese Academy of Sciences, Beijing 100049, (China) , Xisha Zhang Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) University of Chinese Academy of Sciences, Beijing 100049, (China) , Guanxin Zhang Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) and Deqing Zhang *Corresponding author: E-mail Address: [email protected]; E-mail Address: [email protected]; Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, (China) University of Chinese Academy of Sciences, Beijing 100049, (China) https://doi.org/10.31635/ccschem.019.201900056 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Multi-stimuli-responsive field-effect transistors (FETs) with organic/polymeric semiconductors have received increasing attention. Herein, we report a novel strategy for fabricating multi-stimuli-responsive polymeric semiconductors through the incorporation of spiropyran (SP) groups in the polymer side chains. The semiconducting performances of resultant FETs with a diketopyrrolopyrroles (DPP)-based conjugated donor–acceptor (D–A) polymer, that entails SP units in the side chains ( pDSP-1), could be modulated reversibly through UV and visible light irradiations, or UV light irradiation and heating, or acid treatment and heating. Our studies reveal that during the reversible transformations of closed, open, and the protonated forms of spiropyran, achieved by light irradiations, heating, and under acidic conditions, a large dipole moment changes occur, which triggers the reversible variation of semiconducting performance of the FETs. Download figure Download PowerPoint Introduction In the last few decades, the rapid developments of organic and polymeric semiconductors with high charge mobilities have been witnessed.1–5 Newly conjugated molecules and polymers have been designed, synthesized, and new device fabrication methods have been proposed in order to improve the semiconducting performance of organic semiconductors.6–20 Thin-film field-effect transistors (FETs) based on organic/polymeric semiconductors have shown potential applications in memory devices, flexible displays, and wearable electronics.21–24 In recent years, FETs with external-field tunable electrical properties other than electrical voltage have been of considerable interest.25–40 Since the output current could be controlled simultaneously and independently by voltage, as well as other external fields, it is possible to integrate complex functionalities, such as memory devices, switches, and complex logic operations, in a single FET device.41–43 Often, light is used as the external stimuli for optically tunable FETs.22–40 In fact, their fabrications have been achieved by utilizing photochromic molecules, able to undergo reversible light-induced interconversions between two isomers.41 For instance, Samorì and co-workers25 blended diarylethene derivative with P3HT [poly(3-hexylthiophene)] to generate a semiconducting film, which showed optically reversible FET with characteristic bi-stable states upon alternating UV and visible light irradiations. Additionally, Nuckolls and co-workers26 built optically reversible FETs using spiropyrans (SP) in the dielectric layer. Further, recent work by our lab and others27 incorporated azobenzene groups into the flexible alkyl side chains of a conjugated polymer, which was used successfully to fabricate optically tunable FETs with rapid photoresponsiveness. In addition to light, other external inputs, such as heating and introducing chemical species are also of high interest in the generation of multi-stimuli-responsive FETs.44–46 Herein, we report the fabrication of multi-stimuli-responsive FETs with the semiconducting D–A polymer (diketopyrrolopyrrole (DPP)-quaterthiophene conjugated polymer) with the substituted photochromic SP groups in the side chains ( pDSP-1), shown in Scheme 1. The incorporation of SP into the D–A polymer was based on the following considerations: (1) The closed-form, SP, should transform reversibly into the open-form, merocyanine (MC), and the protonated merocyanine (MCH+) forms, after UV or visible light irradiations, heating, addition of acid, and even the application of mechanical force.47,48 (2) By comparison, the open-form, MC, must possess a larger dipole moment (∼17.7 D) than the untransformed, closed-SP-form (∼4.3 D).49 Such a change in the dipole moment, presumably, generates carrier scattering sites within the semiconducting thin film and; as a result, the charge transporting behavior should be reduced. Thus, this became the basis of our fabrication of multi-stimuli-responsive FETs coupled with pDSP-1. Beside, pDSP-1 is a typical diketopyrrolopyrroles (DPP)-based conjugated donor–acceptor (D–A) polymer, which is known for its high charge mobility and good photostability.50–54 In pDSP-1, the SP moieties were incorporated into the side chains through covalent bonding; accordingly, the SP units were distributed uniformly within the semiconducting layer, and thus, the stability of the resulting device was enhanced. In comparison, SP was used in constructing responsive FETs via physical blending with the semiconducting polymer or by interfacial/dielectric modifications.26,30–35 The results revealed that the current and performance of the thin FETs coupled with pDSP-1 device could be reversibly tuned in a relatively faster manner by alternating UV and visible light irradiations, or by UV light irradiation, followed by heating, or by addition of acid, followed by heating. Scheme 1 | Chemical structure of pDSP-1 and the schematic representation of the multi-stimuli-responsive FET architecture. For comparative mechanistic studies, pDSP-5, containing different contents of SP groups in the side chains, was also prepared. Download figure Download PowerPoint Results and Discussion Synthesis and characterization of pDSP-1 The SP side chains conjugated DPP-based D–A polymer, pDSP-1, was prepared by Stille co-coupling of compounds 3 (with SP-containing linear alkyl chains) and 4 (with branching alkyl chains) with the bis(stannyl) compound 5 (see ). The molar ratio of compounds 3 and 4 was set at 1∶1 to enable the formation of the polymer pDSP-1, in which the molar ratio of the linear chains with SP units versus the branching alkyl chains is 1∶1. For comparison (see below), the polymer, pDSP-5, for which the molar ratio of the linear chains with SP units versus the branching alkyl chains was decreased to 1∶5, was prepared under similar conditions. After purification, pDSP-1 and pDSP-5 were obtained in 80% and 87% yields, respectively. pDSP-1 and pDSP-5 were characterized by 1H NMR, solid 13C NMR, and by elemental analysis. The molar ratios of SP-containing side chains versus branching side chains in pDSP-1 and pDSP-5 were calculated to be ∼1.09∶1 and ∼1∶4.67, being close to the respective feed ratios, based on the elemental analysis data. The number averaged molecular weights (Mns) and polydispersities (PDIs) of pDSP-1 and pDSP-5, determined by high-temperature gel permeation chromatography in o-dichlorobenzene at 140 °C, were 16.8 kg mol−1 and 2.9, and 26.0 kg mol−1 and 1.4, respectively. pDSP-1 and pDSP-5 could be dissolved in 1,1,2,2-tetrachloroethane (TeCA) to reach a concentration of ∼5 mg mL−1 at 100 °C. They could not dissolve in chloroform and nonhalogenated solvents, such as toluene and tetrahydrofuran (THF). Both pDSP-1 and pDSP-5 were thermally stable with decomposition temperatures of 309 °C and 371 °C (at a weight loss of 5%) based on thermogravimetric analysis (TGA) data (). Apparently, the incorporation of SP units in the side chains did not affect the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels of pDSP-1 and pDSP-5. On the basis of cyclic voltammograms obtained from cyclic voltammetry (CV) analysis (), the HOMO/LUMO energy levels of pDSP-1 and pDSP-5 were estimated to be −5.18/−3.69 and −5.22/−3.67 eV, respectively (), being close to those of DPP-based D–A polymers reported previously.55–57 Absorption spectral variations for pDSP-1 upon light irradiations, heating, and protonation The SP groups in pDSP-1 were expected to switch reversibly between the closed-form (SP) and the opened form (MC) upon UV and visible light irradiations. As shown in , new absorption peaks in the range of 500–600 nm appeared in the UV-vis absorption spectrum after UV light irradiation of a solution of pDSP-1, and these peaks disappeared after further application of visible light irradiation or heating the solution. Similarly, new absorptions peaks ∼420 nm were observed for the solution of pDSP-1 after the addition of trifluoroacetic acid (TFA or CF3COOH) (see ), and these absorptions disappeared after heating the solution further. We attributed the absorption at ∼420 nm to the formation of the MCH+ form, based on previous reports.47,48 Additionally, such absorption spectral changes could be reversibly repeated for 5 cycles. Moreover, the correlation of the absorption spectral variations with the transformations of SP into MC or MCH+ forms also matched with the data obtained for thin films of pDSP-1. Figure 1a shows the absorption spectra of thin film of pDSP-1 before and after UV light irradiation at 365 nm for 30 s, followed by exposure to visible light irradiation at 470 nm for 10 min. A new absorption at ∼590 nm emerged after UV light irradiation. According to previous studies,47,48 the absorption from 550 to 600 nm is due to the formation of the MC form. Furthermore, visible light irradiation resulted in the gradual decrease of the absorption at 590 nm (see Figure 1b). Visible light irradiation for 10 min was required for the complete disappearance of the absorption due to the MC form. In contrast, this 590 nm peak due to the MC form disappeared quickly after heating the thin film of pDSP-1 at 80 °C for 2 min, as shown in Figure 1c. Indeed, the reversible variation of the absorption at 590 nm for thin film of pDSP-1 could be repeated for five cycles after alternating UV light irradiation and heating at 80 °C, as depicted in Figure 1d. These results indicate that the reversible interconversion between the SP form in pDSP-1 and MC form occurs at fast rates upon UV light irradiation and heating at 80 °C. Moreover, new absorption around 420 nm appeared after the thin film of pDSP-1 was treated with the vapor of TFA (500 ppm for 1 min), and this new peak vanished after further heating at 80 °C for 5 min (Figure 1e,f). We used monomer 3 (see ) as a model for pDSP-1 and pDSP-5 to estimate the conversion ratio by 1H NMR. The 1H NMR spectra of monomer 3 and those after UV light irradiation and treatment with CF3COOD are shown in , respectively. The chemical shifts around 8.65–8.60 ppm, 7.79–7.58, and 6.67–6.64 ppm were due to the MC form (see ), whereas the chemical shifts around 8.50 ppm, 8.29–8.16 ppm, and 7.64 ppm were attributable to the MCH+ form. Based on these 1H NMR data, ∼39.5% of SP units were converted to the MC form after UV irradiation at 365 nm for 30 s. Similarly, ∼82.5% SP units were transformed into the MCH+ moieties after addition of 0.03 M deuterium TFA (d-TFA or CF3COOD). However, the estimated conversion ratios might not reflect the conversion efficiencies of SP into MC (after UV light irradiation) and SP into MCH+ (after treatment with CF3COOH) in thin films of pDSP-1 and pDSP-5. This is because the structures of monomer 3 and pDSP-1/ pDSP-5 are different; thus, their conversion efficiencies are expected to be different both in solution and as thin films. Figure 1 | UV-vis absorption spectra of thin film of pDSP-1: (a) After successive UV light (365 nm for 30 s) and visible light (470 nm for 10 min) irradiations for five cycles. (b) The reversible variations of absorption intensity at 590 nm after alternating UV and visible irradiations. (c) After successive UV light irradiation (365 nm for 30 s) and heating (80 °C for 2 min) for five cycles. (d) The reversible variations of absorption intensity at 590 nm after alternating UV and heating treatments. (e) After successive acid [(CF3COOH) vapor (500 ppm) for 1 min] treatment and heating (80 °C for 5 min) for five cycles. (d) The reversible variations of absorption intensity at 420 nm after alternating acid and heating treatments. Download figure Download PowerPoint Multi-stimuli-responsive FETs We first explored the semiconducting performances of pDSP-1 by fabrication of bottom-gate, bottom-contact (BGBC) FETs before UV light irradiation. Predictably, a thin film of pDSP-1 exhibited a typical p-type semiconducting behavior under ambient conditions based on the transfer and output curves (). As shown in , hysteresis was minimal for the transfer and output characteristics. The maximum/average saturated and linear hole mobilities of thin films of pDSP-1 were measured to be 0.20/0.16 and 0.09/0.07 cm2 V−1 s−1, respectively, after thermal annealing at 160 °C, with on/off ratio of 106–107 (). We further studied the tunable semiconducting properties of FETs with thin films of pDSP-1 after UV light irradiation and heating at 80 °C. Figure 2a and show the transfer and output curves after UV light irradiation at 365 nm for 30 s, followed by heating at 80 °C for 2 min. Undoubtedly, the transfer curve was altered upon UV light irradiation, due to the transformation of the SP moieties of pDSP-1 to MC. Upon heating, the transfer curve was restored, attributable to the conversion of MC to SP. The output curve shows the same tendency (), suggesting that the reversible transformation between SP and MC forms. Such reversible tuning could be repeated for 5 cycles upon the successive UV light irradiation and heating treatment. As shown in Figure 2b and Table 1, the pristine average IDS was measured to be 6.5 × 10−5 A (VGS = VDS = −60 V), and it decreased to 2.1 × 10−5 A after UV light irradiation for 30 s. The drain current increased almost to the initial value after heating the device for 2 min. Figure 2b depicts the reversible variation of the device current for five cycles after alternating UV light irradiation and heating. In addition, the average hole mobility of pDSP-1 thin film could also be reversibly tuned between 0.16 and 0.05 cm2 V−1 s−1. Interestingly, the variation of IDS after UV light irradiation and heating matched well with the variation of the absorption spectra change of pDSP-1 in association with the transformation of the SP and MC forms of SP units in pDSP-1 (Figure 2b). Figure 2 | UV-vis absorption spectra showing: (a) variations in the transfer curves based on different pDSP-1 treatments after alternating UV light irradiation (365 nm for 30 s) and heating (80 °C for 2 min) and (b) the reversible modulation of IDS (VDS = VGS = −60 V) and absorption intensity at 590 nm for five UV/heating cycles. W = 1440 μm, L = 5 μm. I is the drain current upon UV or thermal treatment, while I0 is the initial drain current. A is the absorbance upon UV or thermal treatment, while A0 is the initial absorbance. Download figure Download PowerPoint Table 1 | Average Mobilities (μh), Drain Currents, and Current Variation Ratios for pDSP-1- and pDSP-5-Based FET Devices After UV Light Irradiation or Acid Treatment Polymer μh/cm2 V−1 s−1 I0 Current (10−5 A) Stimulus μh/cm2 V−1 s−1 I Current (10−5 A) Current Variation Ratio (%) pDSP-1 0.16a 6.5a UV 0.05b 2.1b 67d Acid 0.06c 2.3c 65d pDSP-5 0.60a 17.5a UV 0.30b 10.0b 43d Acid 0.28c 9.5c 46d aInitial average mobilities and drain currents (VDS = VGS = −60 V) of pDSP-1 and pDSP-5; baverage mobilities and drain currents after UV irradiations; caverage mobilities and drain currents after acid treatment; dcurrent variation ratio = (I0–I)/I0. We also explored the reversible variations of the semiconducting performances of FETs coupled with pDSP-1 upon either alternating UV and visible light irradiations or alternating acid treatment and heating at 80 °C. As discussed above, the device current and the hole mobility of the thin film of pDSP-1 decreased after UV light irradiation. As shown in Figure 3a,b and , the transfer and output curves for FETs that were treated with UV light, were almost restored to their respective initial ones after further irradiation with visible light at 470 nm for 10 min. Accordingly, the device current (IDS at VGS = VDS = −60 V) was restored to the initial value after further visible light irradiation. Figure 3 | UV-vis absorption spectra showing: (a) variation of transfer curves based on pDSP-1 after alternating UV light (365 nm for 30 s) and visible light (470 nm for 10 min) irradiations; (b) the reversible modulation of IDS (VDS = VGS = −60 V) and absorption intensity at 590 nm for five UV/vis cycles. (c) Variation of transfer curves based on pDSP-1 after alternating acid (500 ppm for 1 min) and heating (80 °C for 5 min) treatment; (d) the reversible modulation of IDS (VDS = VGS = −60 V) and absorption intensity at 420 nm for five acid/heating cycles. W = 1440 μm, L = 5 μm. I is the drain current upon external stimuli treatment, while I0 is the initial drain current. Similarly, A is the absorbance upon external stimuli treatment, while A0 is the initial absorbance. Download figure Download PowerPoint As discussed earlier, and also, displayed in Scheme 1 and Figure 1e, the SP units in pDSP-1 could be transformed into the protonated MC (MCH+) units after exposure to CF3COOH vapor, and the MCH+ moieties, in turn, could switch back to SP upon heating. For this reason, we investigated the variation of semiconducting performances of FETs- pDSP-1 after exposure to the vapor of CF3COOH and heating at 80 °C. First, BGBC FETs were treated with CF3COOH vapor at a concentration of 500 ppm for 1 min, followed by thermal treatment of the devices at 80 °C for 5 min. The transfer and output curves were measured separately before the treatments and after exposure to CF3COOH vapor and heating at 80 °C. As shown in Figure 3c,d, and , both transfer and output curves were varied after exposure to CF3COOH vapor, but the curves were almost restored after further thermal treatment. As listed in Table 1, the average device current (IDS) decreased from 6.5 × 10−5 to 2.3 × 10−5 A with a resultant current variation ratio [(I0−I)/I0] of 65%. After heating at 80 °C, the device current reincreased to 6.5 × 10−5 A. Such current variation could be repeated reversibly by alternating exposures to CF3COOH vapor and thermal treatment. Beside, for drain current, the variation of threshold voltages (Vths) was also observed, as shown in . For instance, Vth for the BGBC FET with pDSP-1 was varied from −5 to −3 V to −10 to −8 V after UV light irradiation. The reversible variation of the semiconducting performance was also observed by treatments with other acids, such as acetic acid (CH3COOH) and hydrochloric acid (HCl), followed by heating, as shown in . These results demonstrated that the semiconducting performances of FETs with pDSP-1 entailing SP units in the side chains could tune reversibly not only by alternating UV light irradiation, visible light irradiation or heating but also by treatment with CF3COOH vapor and heating. Notably, the transfer curves of FETs with pDSP-1 in which the SP units transformed into MC forms could be restored gradually after the devices were left in air for 2 h (see ). This also applied to FETs with pDSP-1 in which the SP units could convert into the MCH+ forms, as depicted in . These results could be attributable to both MC and MCH+ forms able to switch back to the SP forms at room temperature. Thus, the incorporation of SP units in the side chains of pDSP-1 enabled the semiconducting polymer to be multi-stimuli-responsive; accordingly, multi-stimuli-responsive FETs could be fabricated via coupling with such stimuli-responsive semiconducting polymer. Mechanism of FETs/pDSP-1 reversible modulation We further investigated the mechanism of the reversible modulation of the semiconducting performance of FETs with pDSP-1, which is associated with the reversible transformation among SP, MC, and MCH+ upon light irradiations, heating, and acid treatment. We first examined whether the transformation of SP units in pDSP-1 into MC units could influence the interchain packing, thin-film crystallinity, and morphology. We performed geometry measurements by grazing incidence wide-angle X-ray scattering (GIWAXS) on thin film of pDSP-1 untreated and external stimuli-treated samples. The GIWAXS patterns collected for thin film of pDSP-1 before and after UV light irradiation for 30 s and subsequent heating at 80 °C for 2 min are shown in . Our results showed that the 2D-GIWAXS patterns and the corresponding line-cut profiles of thin film of pDSP-1 after UV light irradiation and subsequent heating were almost the same as that of the untreated thin-film (). Both lamellar, including (100), (200), (300), and (400) peaks, and π–π stacking (010) signals for the pristine thin film of pDSP-1 were almost identical to those of the respective thin film after sequential treatments with UV light and heating (). shows the atomic force microscopy (AFM) height images for the untreated spin-coated thin film of pDSP-1 and those after UV light irradiation, followed by heating. The results also revealed no distinct differences in thin-film morphology, consistent with the root-mean-square roughness (RRMS), which remained unchanged for the thin film of pDSP-1 after treatments with UV light and heating. These studies demonstrated that thin-film crystallinity and were not by the transformation of SP units in pDSP-1 into the MC We also measured the of the pristine thin film of pDSP-1 and those after exposure to CF3COOH vapor and sequential thermal treatment at 80 °C (see ) and obtained similar the thin-film of pDSP-1 was also not further that its geometry was not by the treatments. the transformation of SP units in pDSP-1 into the MC units is associated with variations of the dipole moment, that the dipole moment of the SP form is whereas the MC form a dipole moment of In fact, as shown in Figure the for thin film of pDSP-1 were measured to be and before and after UV light irradiations, respectively. Such variation is consistent with the MC form a larger dipole moment than the SP form. is that the MCH+ form, achieved by treatment of the SP form with as the also possess a large dipole According to previous species such as MC and with large dipole might as charge carrier scattering sites by the which to decrease of the FETs device We also observed threshold voltage changes (), which might be due to the variations of charge or semiconducting and we could the reversible tuning of the semiconducting performances of FETs coupled with pDSP-1 to the dipole moment change among SP, MC or MCH+ forms, as in Figure Figure 4 | UV-vis absorption spectra the reversible of based on thin-film of pDSP-1 under UV light irradiation (365 nm for 30 s) and heating treatment (80 °C for 2 Download figure Download PowerPoint Figure 5 | mechanism for multi-stimuli-responsive FETs based on SP-containing semiconducting polymer. Download figure Download PowerPoint further that SP moieties of pDSP-1 are for the charge the polymer pDSP-5 of pDSP-1, Scheme in which the molar ratio of the SP-containing alkyl chains versus the branching ones is 1∶5, was prepared. Thus, polymer pDSP-5 contents of SP units than pDSP-1. After to we that the semiconducting performances of FETs coupled with pDSP-5 could also be tuned reversibly by light irradiations, and heating in the same as FETs with pDSP-1 (see ). However, as shown in Table 1, after UV light irradiation for 30 s, the average device current (IDS at VGS = VDS = −60 V) of pDSP-5 decreased to of the initial current. This further upon exposure of FETs coupled with pDSP-5 to UV and light irradiation, during which the current variation ratio to than that for the FETs device coupled with pDSP-1 under the same conditions. On the other the device current variation ratio exhibited similar after treatment with CF3COOH for FETs coupled with pDSP-5 and pDSP-1. Additionally, as shown in , the for thin film of pDSP-5 were measured to be and before and after UV light irradiations, respectively, and to be than the respective pDSP-1 (see Figure Thus, we from this comparative analysis because pDSP-5 a of SP units than pDSP-1, this resulted in the formation of of MC or MCH+ units after UV light irradiation or treatment with CF3COOH the charge within the thin film of pDSP-5 was by the UV light irradiation or treatment with CF3COOH vapor, with pDSP-1. We report a novel strategy for the of multi-stimuli-responsive polymeric semiconductors through the of SP groups in the side chains of a DPP-based D–A polymer by the of the reversible transformation of SP into MC or MCH+ under light irradiations, and heating. The semiconducting performances of FETs coupled with pDSP-1 that SP units could tune reversibly after alternating UV and visible light irradiations, alternating UV light irradiation, and heating, or alternating protonation with CF3COOH and heating. it was to fabricate multi-stimuli-responsive FETs with such semiconducting SP units in the side chains. Our current results that the molecular transformations among SP, MC, and MCH+ in the side chains of conjugated polymers could affect the charge within the semiconducting thin films. Accordingly, our studies new for applications of semiconducting is of are no

In this study, the environmental dependence of the electrical stability of solution-processed n-channel zinc tin oxide (ZTO) thin-film transistors (TFTs) is reported. Under a prolonged negative gate bias stress, a negative shift in threshold voltage occurs in atmospheric air, whereas a negligible positive shift in threshold voltage occurs under vacuum. In the positive bias-stress experiments, a positive shift in threshold voltage was invariably observed both in atmospheric air and under vacuum. In this study, the negative gate-bias-stress-induced instability in atmospheric air is explained through an internal potential in the ZTO semiconductor, which can be generated owing to the interplay between H2O molecules and majority carrier electrons at the surface of the ZTO film. The positive bias-stress-induced instability is ascribed to electron-trapping phenomenon in and around the TFT channel region, which can be further augmented in the presence of air O2 molecules. These results suggest that the interaction between majority carriers and air molecules will have crucial implications for a reliable operation of solution-processed ZTO TFTs.