InAs Nanocrystals with Robust p‐Type Doping

ConspectusColloidal nanocrystals (NCs) are composed of inorganic cores and organic or inorganic ligand shells and serve as building blocks of NC assemblies. Metal and semiconductor NCs are well known for the size-dependent physical properties of their cores. The large NC surface-to-volume ratio and the space between NCs in assemblies places significant importance on the composition of the NC surface and ligand shell. Nonaqueous colloidal NC syntheses use relatively long organic ligands to control NC size and uniformity during growth and to prepare stable NC dispersions. However, these ligands create large interparticle distances that dilute the metal and semiconductor NC properties of their assemblies. In this Account, we describe postsynthesis chemical treatments to engineer the NC surface and design the optical and electronic properties of NC assemblies. In metal NC assemblies, compact ligand exchange reduces the interparticle distance and drives an insulator-to-metal transition tuning the dc resistivity over a 1010 range and the real part of the optical dielectric function from positive to negative across the visible-to-IR region. Juxtaposing NC and bulk metal thin films in bilayers allows the differential chemical and thermal addressability of the NC surface to be exploited in device fabrication. Ligand exchange and thermal annealing densifies the NC layer, creating interfacial misfit strain that triggers folding of the bilayers and is used to fabricate, with only one lithography step, large-area 3D chiral metamaterials. In semiconductor NC assemblies, chemical treatments such as ligand exchange, doping, and cation exchange control the interparticle distance and composition to add impurities, tailor stoichiometry, or make entirely new compounds. These treatments are employed in longer studied II-VI and IV-VI materials and are being developed as interest in III-V and I-III-VI2 NC materials grows. NC surface engineering is used to design NC assemblies with tailored carrier energy, type, concentration, mobility, and lifetime. Compact ligand exchange increases the coupling between NCs but can introduce intragap states that scatter and reduce the lifetime of carriers. Hybrid ligand exchange with two different chemistries can enhance the mobility-lifetime product. Doping increases carrier concentration, shifts the Fermi energy, and increases carrier mobility, creating n- and p-type building blocks for optoelectronic and electronic devices and circuits. Surface engineering of semiconductor NC assemblies is also important to modify device interfaces to allow the stacking and patterning of NC layers and to realize excellent device performance. It is used to construct NC-integrated circuits, exploiting the library of metal, semiconductor, and insulator NCs, to achieve all-NC, solution-fabricated transistors.

Tuning of the electronic properties of presynthesized colloidal semiconductor nanocrystals (NCs) by doping plays a key role in the prospect of implementing them in printed electronics devices such as transistors and photodetectors. While such impurity doping reactions have already been introduced, the understanding of the doping process, the nature of interaction between the impurity and host atoms, and the conditions affecting the solubility limit of impurities in nanocrystals are still unclear. Here, we used a postsynthesis diffusion-based doping reaction to introduce Ag impurities into InAs NCs. Optical absorption spectroscopy and analytical inductively coupled plasma mass spectroscopy (ICP-MS) were used to present a two-stage doping model consisting of a "doping region" and a "growth region", depending on the impurity to NC ratio in the reaction vessel. X-ray absorption fine-structure (XAFS) spectroscopy was employed to determine the impurity location and correlate between the structural and electronic properties for different sizes of InAs NCs and dopant concentrations. The resulting structural model describes a heterogeneous system where the impurities initially dope the NC, by substituting for In atoms near the surface of the NC, until the "solubility limit" is reached, after which the rapid growth and formation of metallic structures are identified.

We investigate the electronic properties of individual n-type (Cu) doped and p-type (Ag) doped InAs colloidal nanocrystals (NCs) in the 2–8 nm size range from their charge transfers toward a highly oriented pyrolytic graphite (HOPG) substrate, using ultrahigh vacuum Kelvin probe force microscopy (KPFM) with elementary charge sensitivity at 300 K. The NC active dopant concentration is measured as ND = 8 × 1020 cm–3 and NA > 5 × 1020 cm–3 for n- and p-type doping, respectively. The electrostatic equilibrium between the NC and the HOPG reference substrate is investigated and reveals an enhancement of the Fermi-level mismatch between the NCs and the HOPG substrate at reduced NC sizes, both for n- and p-type doping. It also shows, for n-type doped NCs with smallest sizes (∼2 nm), the existence of a full depletion regime, in which smallest NCs contain single ionized dopants. Results are compared with self-consistent tight-binding calculations of the electronic structure of InAs NCs, including hydrogenoid impurities and the presence of a host substrate, in the case of n-type doped NCs. The observed enhancement of the NC–HOPG Fermi-level mismatch can be understood by considering a size-dependent electrostatic contribution attributed to dipolar effects at the NC–ligand interface. The estimated surface dipole density equals a few Debye/nm2 and is increased at smallest NC sizes, which follows the enhancement of ligand densities at small NC sizes previously reported for metallic or semiconducting NCs. The results put forward the role played by the NC–ligand interface electrostatics for electronic applications.

Sputtered transparent electrodes for optoelectronic devices: Induced damage and mitigation strategies

The work presented in this thesis deals with optical studies of semiconductor quantum dots (QDs) in the InGaAs material system. It is shown that for self-assembled InAs QDs, the interaction with the surrounding GaAs barrier and the InAs wetting layer (WL) in particular, has a very large impact on their optical properties. The ability to control the charge state of individual QDs is demonstrated and attributed to a modulation in the carrier transport dynamics in the WL. After photo-excitation of carriers (electrons and holes) in the barrier, they will migrate in the sample and with a certain probability become captured into a QD. During this migration, the carriers can be affected by exerting them to an external magnetic field or by altering the temperature. An external magnetic field applied perpendicular to the carrier transport direction will lead to a decrease in the carrier drift velocity since their trajectories are bent, and at sufficiently high field strength become circular. In turn, this decreases the probability for the carriers to reach the QD since the probability for the carriers to get trapped in WL localizing potentials increases. An elevated temperature leads to an increased escape rate out of these potentials and again increases the flow of carriers towards the QD. These effects have significantly different strengths for electrons and holes due to the large difference in their respective masses and therefore it constitutes a way to control the supply of charges to the QD. Another effect of the different capture probabilities for electrons and holes into a QD that is explored is the ability to achieve spin polarization of the neutral exciton (X0). It has been concluded frequently in the literature that X0 cannot maintain its spin without application of an external magnetic field, due to the anisotropic electron – hole exchange interaction (AEI). In our studies, we show that at certain excitation conditions, the AEI can be by-passed since an electron is captured faster than a hole into a QD. The result is that the electron will populate the QD solely for a certain time window, before the hole is captured. During this time window and at polarized excitation, which creates spin polarized carriers, the electron can polarize the QD nuclei. In this way, a nuclear magnetic field is built up with a magnitude as high as ~ 1.5 T. This field will stabilize the X0 spin in a similar manner as an external magnetic field would. The build-up time for this nuclear field was determined to be ~ 10 ms and the polarization degree achieved for X0 is ~ 60 %. In contrast to the case of X0, the AEI is naturally cancelled for the negatively charged exciton (X-) and the positively charged exciton (X+) complexes. This is due to the fact that the electron (hole) spin is paired off in case of X- (X+). Accordingly, an even higher polarization degree (~ 73 %) is measured for the positively charged exciton. In a different study, pyramidal QD structures were employed. In contrast to fabrication of self-assembled QDs, the position of QDs can be controlled in these samples as they are grown in inverted pyramids that are etched into a substrate. After sample processing, the result is free-standing AlGaAs pyramids with InGaAs QDs inside. Due to the pyramidal shape of these structures, the light extraction is considerably enhanced which opens up possibilities to study processes un-resolvable in self-assembled QDs. This has allowed studies of Auger-like shake-up processes of holes in single QDs. Normally, after radiative recombination of X+, the QD is populated with a ground state hole. However, at recombination, a fraction of the energy can be transferred to the hole so that it afterwards occupies an excited state instead. This process is detected experimentally as a red-shifted luminescence satellite peak with an intensity on the order of ~ 1/1000 of the main X+ peak intensity. The identification of the satellite peak is based on its intensity correlation with the X+ peak, photoluminescence excitation measurements and on magnetic field measurements.

Semiconductors in the shape of nanostructures, including quantum dots (0-dimension) and nanoplatelets (2-dimension) demonstrate unique electronic and optoelectronic properties because of the quantum mechanism related to excitons confined in low dimensions. Such confinement can provide intriguing properties like high capability of photo-activated carriers, band gap tunability, splendid photoluminescence (PL) yield, narrow emission bandwidth, high absorption efficiency of light, fast fluorescence lifetime, etc. These properties attract great attention due to their high potential in applications like solar cells, light-emitting diodes (LEDs), lasers, photodetectors, catalysts, etc. Novel semiconductors including MoS2, MXene and perovskite have received increasing research interest in recent years. Unlike traditional semiconductor materials like Si, Ge, II-VI and III-V materials, the novel ones usually have low-dimensional form natively, which have emerged as very promising candidates for advanced electronic and optoelectronic devices. Among the various novel semiconductor materials, this thesis focuses on two types of them, which are MXene Sc2CO2 and cesium lead halide perovskite CsPbBr3. The two materials have some features in common. Both of them are direct band gap materials, which makes them available for optical applications. And they can be synthesized directly into two-dimensional structure, which o ers the opportunity to learn the influence of one-dimensional confinement. To extend the study into another dimension, the quantum dots of CsPbBr3 are also studied. The first part of the thesis focuses on the study of Sc2COx which is a type of semi-conductive MXene. The material is synthesized for the first time by using magnetron sputtering. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are performed to determine the composition of the sample. The band gap of Sc2COx sample is determined by ultraviolet-visible (UV-VIS) range absorption and photoluminescence (PL) spectrum. The Tauc plot indicates that Sc2COx is a direct band gap material. The high sensitivity of scandium to oxygen makes the sample contain both scandium carbide and scandium oxide. Thermal annealing introduces more oxidation into the sample. The structure of MXene will be broken when the annealing temperature is higher than 600C. The second part of the thesis focuses on the study of perovskite CsPbBr3 in forms of nanoplatelets and quantum dots. For CsPbBr3 nanoplatelets, the effect of thickness of cesium lead halide perovskite CsPbBr3 nanoplatelets (NPLs) on their electronic structure and optical properties are investigated using an 8-band k · p model which is based on effective-mass envelope function theory with exciton binding energy consideration. We first reported the CsPbBr3 nanoplatelets' band structure and optical gain with exciton effect. As the thickness of NPLs decreases, their band gap increases and the band mixing is more obvious which influences the transition matrix element (TME). The optical gain of CsPbBr3 nanoplatelets is calculated by taking into account TME, Fermi factor, injected carrier density and thickness. A blue shift of peak position in optical gain can be observed as the thickness of NPLs decreases. For any given NPL with a certain thickness, there is a slight blue shift in the peak position of optical gain as the carrier density increases because of band filling effect. The maximum optical gain of thinner NPLs needs higher carrier density to reach saturation. To obtain high differential gain, the carrier density in thick NPLs has to maintain in a small range. Experimental work is carried out and the results agree with our theoretical results very well. For quantum dot, the size effect on the electronic structure and optical properties of cubic CsPbBr3 perovskite quantum dots are investigated by using an 8-band k · p model which is also based on effective-mass envelope function theory with exciton binding energy consideration. Quantum dots with smaller sizes have larger band gaps due to quantum confinement. The transition matrix element (TME) is also influenced by the size. The optical gain of such material is calculated by considering TME, Fermi factor, injected carrier density, quantum dot size, and dephasing rate. A higher density of the injected carrier is needed for smaller quantum dots to get a positive optical gain. The peak of optical gain has a blue shift as the size of a quantum dot decreases. When the carrier density increases for a quantum dot with a certain size, a blue shift in emission peak position can be observed due to the band filling effect. Smaller quantum dots have higher differential optical gain at carrier densities within a certain range. In a word, two direct band gap semiconductor materials, Sc2COx and CsPbBr3 are studied via theoretical and experimental approaches. Their band-structure and optical properties are presented, followed by the analysis of their potential in applications.

Laser ablation of Si(100) targets immersed in deionized water at room temperature was studied by applying the second harmonic of a ns pulsed Nd:YAG laser and by employing different methods for characterization of the resulting nanocrystals (NCs). Spontaneous Raman scattering showed first order Raman spectra of the Si NCs, which varied in their Raman shift and width. The Raman data were analyzed using a phonon confinement model, involving three-dimensional confinement and lognormal size distributions for the nanocrystallites constituting the samples, indicating the presence of particles greater than ∼2 nm diameter. High-resolution transmission electron microscopy and electron diffraction of the as-prepared samples showed NCs with diameters greater than ∼1.75 nm and quantum dots, corresponding to Si/SiOx. The optical properties of the Si NCs were studied with cathodoluminescence (CL) spectroscopy for sample temperatures in the 50–300 K range, which exhibited a supercontinuum emission ranging from the near ultraviolet to the red regions. The emission is characterized by four major emission bands, peaked at 1.88, 2.18, 2.64 and 3.44 eV, which practically remain at the same positions as the temperature is varied. These bands are assigned to excitonic recombination in the Si NCs, whose electron/hole transition energies are consistent with the quantum confinement of carriers in four size-dispersed groups of Si NCs. The influence of electron beam irradiation on the passivation of the NCs was examined, showing quenching of the CL as a function of electron-beam exposure time. This quenching is attributed to desorption of hydrogen, disruption of the NC surface passivation, and formation of nonradiative recombination channels involving surface states in the gap. The approach used in this study demonstrates the possibility of growing nano-scale Si crystals with the potential for incorporation into optoelectronic devices or for use in biomedical applications.

This thesis describes the study of the development and device physics of organic field-effect transistors and organic light-emitting field-effect transistors. Using chemical and physical properties of organic semiconductors such as light emission, multi-functional devices, or “Active Channel Field-Effect Transistors” can be created. This thesis first describes the creation of basic ambipolar FETs before building on this knowledge to develop ambipolar as well as hole-dominated light emitting field-effect transistors. Ambipolar FETs using a diketopyrrolopyrrole-based copolymer were first investigated in this thesis. When gold electrodes were used, the devices could transport only holes. The charge injection was altered by changing the contacts to aluminium. The devices then were able to conduct both electrons and holes with maximum electron and hole mobilities of order 10−4 cm2 V−1 s−1 and 10−3 cm2 V−1 s−1 respectively. Light-emitting field-effect transistors were first studied using the model emissive polymer Super Yellow. Charge injection was studied by altering the electron-injecting contact. Materials used included the low work function metals Ca, Ba and Sm, the inorganic salt Cs2CO3 and the organic material TPBi. Ambipolar devices were created using a neat Super Yellow layer. Electron and hole mobilities in these devices were both of the order of 10−4 cm2 V−1 s−1. The maximum external quantum efficiency was 1.2 ± 0.2 % with a Sm electron-injecting contact, however this occurred at a brightness of less than 1 cd m−2. A maximum brightness of 43 ± 9 cd m−2 was obtained using a Cs2CO3-Ag electron-injecting contact in electron accumulation mode. At this brightness, the external quantum efficiency was 0.19 ± 0.04 %. Unipolar devices were also fabricated by adding high mobility charge transport layer of PBTTT underneath the Super Yellow. Since PBTTT had a hole mobility that was orders of magnitude greater than the charge carrier mobilities of Super Yellow, charges were transported in the PBTTT, and recombined within the Super Yellow layer. Super Yellow acted as an emissive layer only. Using this bilayer device architecture, a maximum brightness of 100 ± 4 cd m−2 was obtained with a Ca electron-injecting contact. An external quantum efficiency of 0.04 ± 0.01 % was achieved at the maximum brightness. In order to improve the external quantum efficiency of LEFETs, phosphorescent materials were chosen for use as the emissive layer. A co-evaporated layer of the iridium complex Ir(ppy)3 in a host of CBP was initially used in the devices. Charge injection was studied by using either a Ba or a TPBi/Ba electron-injecting contact. Devices were first fabricated using PBTTT as a hole-dominated charge transport layer. Using a TPBi/Ba electron-injecting contact, the maximum brightness achieved was 200 ± 40 cd m−2 with an external quantum efficiency of 0.18 ± 0.01 % at the maximum brightness. Devices were then fabricated using a charge transport layer of DPP-DTT to create ambipolar devices. Electron and hole mobilities in these devices were both of order 10−2 cm2 V−1 s−1. The maximum external quantum efficiency was 0.4 ± 0.2 % at the maximum brightness of 500 ± 200 cd m−2 in hole accumulation mode using an electron-injecting contact of TPBi/Ba. In order to avoid using a co-deposited emissive layer, a solution-processed iridium-cored dendrimer layer that could be deposited without a host material was used. Like in the previous work, a charge transport layer of PBTTT was deposited underneath the emissive layer. Charge injection in the devices was altered using a change in the device geometry. Non-planar source and drain contacts were used, where one contact was deposited directly on top of the charge transport layer and the other contact on top of the emissive layer. Planar contacts were also used as control devices. There was clear evidence for transport of both holes and electrons when planar contacts were used, with light emission occurring at both contacts. The maximum mobility, brightness and external quantum efficiency was achieved using the non-planar contact geometry. Holes could be directly injected into the charge transport layer, rather than via the emissive layer. The hole mobility in these devices was 3.5 × 10−3 ± 6 × 10−4 cm2 V−1 s−1. The maximum brightness was 140 ± 50 cd m−2 with an external quantum efficiency of 0.11 ± 0.02 % at the maximum brightness. This thesis describes work on the study of charge injection into organic field-effect transistors by changing either the material used for the electron-injecting contact, or by changing the device geometry. This was achieved in devices that were either dominated by holes, or showed evidence of both electron and hole transport within the device. By changing the charge injection into the device, it was possible to improve the charge carrier mobilities, the brightness or the external quantum efficiency. Improving one or more of these things brings light emitting transistors towards becoming a viable technology.

Several experimental works have reported that a single high-energy photon could generate multiple excitons in semiconductor nanocrystals and several theories have been proposed to explain these results. Using a tight-binding method, we calculate the electronic structure of InAs, Si, and PbSe nanocrystals and we investigate two models of the multiple exciton generation (MEG). We show that the impact ionization process is efficient at high energy, with lifetimes as small as $10\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$. The behavior of the impact ionization rate versus the energy is basically the same in all materials in spite of large differences in their electronic structure. We present simulations of the MEG showing that, in PbSe and Si nanocrystals, the impact ionization alone cannot explain the efficiencies measured at high energy, even in the limit where there is no relaxation of the excited carriers by electron-phonon scattering. In InAs nanocrystals, the impact ionization process could only explain the lowest yields reported in the literature. We calculate the spectral densities of multiexciton states and we evaluate the possibility of direct and instantaneous photogeneration of multiexcitons. We confirm the importance of the multiexciton spectral densities in the MEG problem because of their rapid variation over several orders of magnitude as a function of the energy. However, we show that the high MEG efficiencies in PbSe and Si nanocrystals, up to seven excitons per photon, would imply a very efficient relaxation in multiexciton states, whereas they are characterized by a negligible density.

Hybrid field-effect transistors (FETs) based on poly(3-hexylthiophene) (P3HT) containing CdSe quantum dots (QDs) were fabricated. The effect of the concentration of QDs on charge transport in the hybrid material was studied. The influence of the QDs capping ligand on charge transport parameters was investigated by replacing the conventional trioctylphosphine oxide (TOPO) surfactant with pyridine to provide closer contact between the organic and inorganic components. Electrical parameters of FETs with an active layer made of P3HT:CdSe QDs blend were determined, showing field-effect hole mobilities up to 1.1×10−4 cm2/Vs. Incorporation of TOPO covered CdSe QDs decreased the charge carrier mobility while the pyridine covered CdSe QDs did not alter this transport parameter significantly.

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

We report here for the first time the synthesis of LEEH caped very small size (2 nm) ZnTe quantum dots at low temperature (less than 100 °C) using a simple chemical route. The effects of aging and stirring time on the absorption spectra of the quantum dots were investigated. The synthesized nanocrystal (NC) was characterized by PL, TEM, XRD and the formation of very small size quantum dots having FCC structure was confirmed. Further, blue emission from the prepared sample was observed during exposure to monochromatic UV radiation. ZnTe NCs obtained in this study were found to be more stable compared to those presented in literature reports. ZnTe NCs may be considered as a new material in place of CdTe for optoelectronics devices.

This paper presents simulations predicting the feasibility of 9-nm wraparound quantum dot channel (QDC) field-effect transistors (FETs). In particular, II–VI lattice-matched layers which reduce the density of interface states, serving as top (tunnel gate), side, and bottom gate insulators, have been simulated. Quantum simulations show FET operation with voltage swing of ~0.2 V. Incorporation of cladded quantum dots, such as SiOx–Si and GeOx–Ge, under the gate tunnel oxide results in electrical transport in one or more quantum dot layers which form a quantum dot superlattice (QDSL). Long-channel QDC FETs have experimental multistate drain current (ID)–gate voltage (VG) and drain current (ID)–drain voltage (VD) characteristics, which can be attributed to the manifestation of extremely narrow energy minibands formed in the QDSL. An approach for modeling the multistate ID–VG characteristics is reported. The multistate characteristics of QDC FETs permit design of compact two-bit multivalued logic circuits.

Quantum dot materials have the advantages of low cost, wide spectral response and good photostability, and are widely used in optoelectronic fields such as light-emitting diodes and photodetectors. Quantum dots are the key materials for the next generation of new semiconductor optoelectronic devices. The direct integration of nano-quantum dots with silicon-based materials can simplify the fabrication process of optoelectronic devices. The environmental stability of lead selenide (PbSe) quantum dots is poorer than that of lead sulfide (PbS) quantum dots, which limits its application in optoelectronic devices. In this paper, we combine lead selenide quantum dots with silicon-based photovoltaic junction field effect transistors, and introduce halides in the synthesis process to improve the stability of quantum dots in air. We analyze and study the performance of this new type of near-infrared quantum dot photodetector, and study the influence of factors such as channel area and quantum dot size on the performance of the detector, which provides ideas for further optimizing the photodetection performance.

The application of semiconductor nanocrystals (NCs) in optoelectronic devices and photocatalysis inevitably involves them in charged states. The carrier and exciton dynamics of electrochemically charged NCs in solutions have yet to be reported. Herein, the electrochemical charging effects in colloidal CdSe/CdS core/shell quantum dots (QDs) are systematically investigated using static spectroelectrochemistry (SEC) and in situ transient absorption (TA) spectroscopy. Static SEC reveals the presence of in-gap trap states from 0.9eV below the conduction band (CB) edge. Negligible changes in TA spectra and kinetics were observed from open circuit potential (OCP) to more anodic potentials within the QD band gap. At cathodic potentials, the negatively charged QDs show band edge trion decay with a lifetime of 690±31ps and slower 1P to 1S electron relaxation with time constants of 12.4±0.8ps assigned to the spin blockade effect and 316±35ps assigned to the phonon bottleneck effect. Our study reveals rich effects of charging on QD excited state under nearly native conditions.