Hydrogen Incorporation in III‐N‐V Semiconductors: From Macroscopic to Nanometer Control of the Materials’ Physical Properties

The effects of hydrogen incorporation in dilute nitride semiconductors, specifically GaAs_1‐xN_x, are discussed. The remarkable consequences of hydrogen irradiation include tuneable and reversible changes in the electronic, optical, structural, and electrical properties of these materials. The highly trapping‐limited diffusion of H atoms in dilute nitrides results in the formation of extremely sharp heterointerfaces between H‐containing and H‐free regions of the crystals. This, in turn, offers an unprecedented possibility to tailor the physical properties of a semiconductor chip in its growth plane with nanometer precision. A number of examples are presented and discussed.     

Versatile and Highly Efficient Controls of Reversible Topotactic Metal–Insulator Transitions through Proton Intercalation

The ability to tailor a new crystalline structure and associated functionalities with a variety of stimuli is one of the key issues in material design. Developing synthetic routes to functional materials with partially absorbed nonmetallic elements (i.e., hydrogen and nitrogen) can open up more possibilities for preparing novel families of electronically active oxide compounds. Fast and reversible uptake and release of hydrogen in epitaxial ABO₃ manganite films through an adapted low‐frequency inductively coupled plasma technology is introduced. Compared with traditional dopants of metallic cations, the plasma‐assisted hydrogen implantations not only produce reversibly structural transformations from pristine perovskite (PV) phase to a newly found protonation‐driven brownmillerite one but also regulate remarkably different electronic properties driving the material from a ferromagnetic metal to a weakly ferromagnetic insulator for a range of manganite (La_1?xSr_xMnO₃) thin films. Moreover, a reversible perovskite‐brownmillerite‐perovskite transition is achieved at a relatively low temperature (T ≤ 350 °C), enabling multifunctional modulations for integrated electronic systems. The fast, low‐temperature control of structural and electronic properties by the facile hydrogenation/dehydrogenation treatment substantially widens the space for exploring new possibilities of novel properties in proton‐based multifunctional materials.     

Features of a priori heavy doping of the <Emphasis Type="Italic">n</Emphasis>-TiNiSn intermetallic semiconductor

The crystal structure, the distribution of electron density, and the energy, kinetic, and magnetic properties of the n-TiNiSn intermetallic semiconductor are investigated. It is shown that a priori doping of n-TiNiSn with donors originates from partial, up to 0.5 at %, redistribution of Ti and Ni atoms in crystallographic sites of Ti atoms. The correlation is established between the donor concentration, amplitude of modulation of the continuous energy bands, and degree of filling of low-scale fluctuation potential wells with charge carriers. The results obtained are discussed within the Shklovskii-Efros model of a heavily doped and compensated semiconductor.     

Effect of the accumulation of excess Ni atoms in the crystal structure of the intermetallic semiconductor n-ZrNiSn

The crystal structure, electron density distribution, and energy, kinetic, and magnetic properties of the n-ZrNiSn intermetallic semiconductor heavily doped with a Ni impurity are investigated. The effect of the accumulation of an excess number of Ni_{1 + x} atoms in tetrahedral interstices of the crystal structure of the semiconductor is found and the donor nature of such structural defects that change the properties of the semiconductor is established. The results obtained are discussed within the Shklovskii-Efros model of a heavily doped and strongly compensated semiconductor.     

Fabrication of Releasable Single‐Crystal Silicon–Metal Oxide Field‐Effect Devices and Their Deterministic Assembly on Foreign Substrates

A new class of thin, releasable single‐crystal silicon semiconductor device is presented that enables integration of high‐performance electronics on nearly any type of substrate. Fully formed metal oxide–semiconductor field–effect transistors with thermally grown gate oxides and integrated circuits constructed with them demonstrate the ideas in devices mounted on substrates ranging from flexible sheets of plastic, to plates of glass and pieces of aluminum foil. Systematic study of the electrical properties indicates field‐effect mobilities of ≈710 cm² V^?1 s^?1, subthreshold slopes of less than 0.2 V decade^?1 and minimal hysteresis, all with little to no dependence on the properties of the substrate due to bottom silicon surfaces that are passivated with thermal oxide. The schemes reported here require only interconnect metallization to be performed on the final device substrate, which thereby minimizes the need for any specialized processing technology, with important consequences in large‐area electronics for display systems, flexible/stretchable electronics, or other non‐wafer‐based devices.     

Design and Synthesis of “All‐in‐One” Multifunctional FeS2 Nanoparticles for Magnetic Resonance and Near‐Infrared Imaging Guided Photothermal Therapy of Tumors

The ideal theranostic nanoplatform for tumors is a single nanoparticle that has a single semiconductor or metal component and contains all multimodel imaging and therapy abilities. The design and preparation of such a nanoparticle remains a serious challenge. Here, with FeS₂ as a model of a semiconductor, the tuning of vacancy concentrations for obtaining “all‐in‐one” type FeS₂ nanoparticles is reported. FeS₂ nanoparticles with size of ≈30 nm have decreased photoabsorption intensity from the visible to near‐infrared (NIR) region, due to a low S vacancy concentration. By tuning their shape/size and then enhancing the S vacancy concentration, the photoabsorption intensity of FeS₂ nanoparticles with size of ≈350 nm (FeS₂‐350) goes up with the increase of the wavelength from 550 to 950 nm, conferring the high NIR photothermal effect for thermal imaging. Furthermore, this nanoparticle has excellent magnetic properties for T₂‐weighted magnetic resonance imaging (MRI). Subsequently, FeS₂‐350 phosphate buffer saline (PBS) dispersion is injected into the tumor‐bearing mice. Under the irradiation of 915‐nm laser, the tumor can be ablated and the metastasis lesions in liver suffer significant inhibition. Therefore, FeS₂‐350 has great potential to be used as novel “all‐in‐one” multifunctional theranostic nanoagents for MRI and NIR dual‐modal imaging guided NIR‐photothermal ablation therapy (PAT) of tumors.     

CdIn2S4 Nanotubes and “Marigold” Nanostructures: A Visible‐Light Photocatalyst

Nanostructured photocatalysts with high activity are sought for solar production of hydrogen. Spinel semiconductors with different nanostructures and morphologies have immense importance for photocatalytic and other potential applications. Here, a chemically stable cubic spinel nanostructured CdIn₂S₄ prepared by a facile hydrothermal method is reported as a visible‐light driven photocatalyst. A pretty, marigold‐like morphology is observed in aqueous‐mediated CdIn₂S₄, whereas nanotubes of good crystallinity, 25 nm in diameter, are obtained in methanol‐mediated CdIn₂S₄. The aqueous‐ and methanol‐mediated CdIn₂S₄ products show excellent photocatalytic activity compared to other organic mediated samples, and this is attributed to their high degree of crystallinity. The CdIn₂S₄ photocatalyst gives quantum yields of 16.8 % (marigold‐like morphology) and 17.1 % (nanotubes) at 500 nm, respectively, for the H₂ evolution reaction. The details of the characteristics of the photocatalyst, such as crystal and band structure, are reported. Considering the importance of hydrogen energy, CdIn₂S₄ will be an excellent candidate as a catalyst for “photohydrogen” production under visible light. Being a nanostructured chalcogenide semiconductor, CdIn₂S₄ will have other potential prospective applications, such as in solar cells, light‐emitting diodes, and optoelectronic devices.     

p‐Type Beta‐Silver Vanadate Nanoribbons for Nanoelectronic Devices with Tunable Electrical Properties

β‐AgVO₃, as a stable phase and a typical silver vanadium oxide, has performed special ionic and electrical properties. The construction of nanoelectronic devices based on ultralong β‐AgVO₃ nanoribbons (NRs) is reported, including nano‐field‐effect transistor (nano‐FET) and nano‐Schottky barrier diode (nano‐SBD). Owing to the specific channel structure and ion conductivity, the nano‐FET exhibits typical p‐type semiconductor characteristics and the nano‐SBD with Al contacts performs a prominent rectifying behavior with an on/off ratio of up to 10³. Besides, tunable electrical transport properties can be achieved by tailoring the material properties, and it is demonstrated that the bridging NR numbers and diameters have a positive effect on electrical transport properties, while a complex variation trend is observed in the case of surface modification by photo‐irradiation. Electron spin resonance (ESR) spectrum further illuminates that the induced vacancies play an important role on the electrical transport properties of β‐AgVO₃ nanoribbons. Easy access to the ultralong β‐AgVO₃ NRs makes them a promising candidate for potential applications in nanoelectronic devices.     

Excitons and Electron–Hole Liquid State in 2D γ‐Phase Group‐IV Monochalcogenides

Different dispersion near the electronic band edge of a semiconductor can have great influence on its transport, thermoelectric, and optical properties. Using first‐principles calculations, it is demonstrated that a new phase of group‐IV monochalcogenides (γ‐MX, M = Ge, Sn; X = S, Se, or Te) can be stabilized in monolayer limit. γ‐MXs are shown to possess a unique band dispersion—that is, camel's back like structure—in the top valence band. The band nesting effect near the camel's back region induces a large excitonic absorbance and significantly different exciton behaviors from other 2D materials. Importantly, the small effective mass and the indirect characteristics of lowest‐energy exciton render it advantageous for the generation of electron–hole liquid state. After careful evaluation of the electron–hole dissociation temperature and the Mott critical density, it is predicted that a high‐temperature exciton gas to electron–hole liquid phase transition can be achieved in these materials with a low excitation power density. The findings open up new opportunities for both the fundamental research on exciton physics and design of excitonic devices based on 2D materials with distinct band dispersion.     

A Facile Method for the Growth of Organic Semiconductor Single Crystal Arrays on Polymer Dielectric toward Flexible Field‐Effect Transistors

Polymer dielectrics with intrinsic mechanical flexibility are considered as a key component for flexible organic field‐effect transistors (OFETs). However, it remains a challenge to fabricate highly aligned organic semiconductor single crystal (OSSC) arrays on the polymer dielectrics. Herein, for the first time, a facile and universal strategy, polar surface‐confined crystallization (PSCC), is proposed to grow highly aligned OSSC arrays on poly(4‐vinylphenol) (PVP) dielectric layer. The surface polarity of PVP is altered periodically with oxygen‐plasma treatment, enabling the preferential nucleation of organic crystals on the strong‐polarity regions. Moreover, a geometrical confinement effect of the patterned regions can also prevent multiple nucleation and misaligned molecular packing, enabling the highly aligned growth of OSSC arrays with uniform morphology and unitary crystallographic orientation. Using 2,7‐dioctyl[1]benzothieno[3,2‐b]benzothiophene (C8‐BTBT) as an example, highly aligned C8‐BTBT single crystal arrays with uniform molecular packing and crystal orientation are successfully fabricated on the PVP layer, which can guarantee their uniform electrical properties. OFETs made from the C8‐BTBT single crystal arrays on flexible substrates exhibit a mobility as high as 2.25 cm² V^?1 s^?1, which has surpassed the C8‐BTBT polycrystalline film‐based flexible devices. This work paves the way toward the fabrication of highly aligned OSSCs on polymer dielectrics for high‐performance, flexible organic devices.     

Marangoni‐Effect‐Assisted Bar‐Coating Method for High‐Quality Organic Crystals with Compressive and Tensile Strains

Low‐cost solution‐shearing methods are highly desirable for deposition of organic semiconductor crystals over a large area. To enhance the rate of evaporation and deposition, elevated substrate temperature is commonly employed during shearing processes. However, the Marangoni flow induced by a temperature‐dependent surface‐tension gradient near the meniscus line shows negative effects on the deposited crystals and its electrical properties. In the current study, the Marangoni effect to improve the shearing process of 2,7‐dioctyl[1]benzothieno[3,2‐b ][1]benzothiophene for organic field‐effect transistor (OFET) applications is utilized and regulated. By modifying the gradient of surface tension with different combinations of solvents, the mass transport of molecules is much more favorable, which largely enhances the deposition rate, reduces organic crystal thickness, enlarges grain sizes, and improves coverage. The average and highest mobility of OFETs can be increased up to 13.7 and 16 cm² V^?1 s^?1. This method provides a simple deposition approach on a large scale, which allows to further fabricate large‐area circuits, flexible displays, or bioimplantable sensors.     

Schottky‐Barrier‐Controllable Graphene Electrode to Boost Rectification in Organic Vertical P–N Junction Photodiodes

Monolayer graphene is used as an electrode to develop novel electronic device architectures that exploit the unique, atomically thin structure of the material with a low density of states at its charge neutrality point. For example, a single semiconductor layer stacked onto graphene can provide a semiconductor–electrode junction with a tunable injection barrier, which is the basis for a primitive transistor architecture known as the Schottky barrier field‐effect transistor. This work demonstrates the next level of complexity in a vertical graphene–semiconductor architecture. Specifically, an organic vertical p‐n junction (p‐type pentacene/n‐type N,N′‐dioctyl‐3,4,9,10‐perylenedicarboximide (PTCDI‐C₈)) on top of a graphene electrode constituting a novel gate‐tunable photodiode device structure is fabricated. The model device confirms that controlling the Schottky barrier height at the pentacene–graphene junction can (i) suppress the dark current density and (ii) enhance the photocurrent of the device, both of which are critical to improve the performance of a photodiode.     

Benzodithiophene Copolymer—A Low‐Temperature,Solution‐Processed High‐Performance Semiconductor for Thin‐Film Transistors

Poly(4,8‐didodecyl‐2,6‐bis‐(3‐methylthiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene) self‐assembled on appropriate substrates from solution and formed highly structured thin films at low temperatures. As an as‐prepared thin‐film semiconductor without thermal annealing, it exhibited excellent field‐effect transistor properties with mobility of ～ 0.15 cm² V^–1 s^–1 in thin‐film transistors.     

Tuning Optoelectronic Properties of Ambipolar Organic Light‐ Emitting Transistors Using a Bulk‐Heterojunction Approach

Bulk‐heterojunction engineering is demonstrated as an approach to producing ambipolar organic light‐emitting field‐effect transistors with tunable electrical and optoelectronic characteristics. The electron and hole mobilities, as well as the electroluminescence intensity, can be tuned over a large range by changing the composition of a bimolecular mixture consisting of α‐quinquethiophene and N,N′‐ditridecylperylene‐3,4,9,10‐tetracarboxylic‐diimide. Time‐resolved photoluminescence spectroscopy reveals that the phase segregation of the two molecules in the bulk heterojunction and their electronic interaction determine the optoelectronic properties of the devices. The results presented show that the bulk‐heterojunction approach, which is widely used in organic photovoltaic cells, can be successfully employed to select and tailor the functionality of field‐effect devices, including ambipolar charge transport and light emission.     

Conductive “Smart” Hybrid Hydrogels with PNIPAM and Nanostructured Conductive Polymers

Stimuli‐responsive hydrogels with decent electrical properties are a promising class of polymeric materials for a range of technological applications, such as electrical, electrochemical, and biomedical devices. In this paper, thermally responsive and conductive hybrid hydrogels are synthesized by in situ formation of continuous network of conductive polymer hydrogels crosslinked by phytic acid in poly(N‐isopropylacrylamide) matrix. The interpenetrating binary network structure provides the hybrid hydrogels with continuous transporting path for electrons, highly porous microstructure, strong interactions between two hydrogel networks, thus endowing the hybrid hydrogels with a unique combination of high electrical conductivity (up to 0.8 S m^?1), high thermoresponsive sensitivity (significant volume change within several seconds), and greatly enhanced mechanical properties. This work demonstrates that the architecture of the filling phase in the hydrogel matrix and design of hybrid hydrogel structure play an important role in determining the performance of the resulting hybrid material. The attractive performance of these hybrid hydrogels is further demonstrated by the developed switcher device which suggests potential applications in stimuli‐responsive electronic devices.     

Electroless‐Plated Gold Contacts for High‐Performance,Low Contact Resistance Organic Thin Film Transistors

Deposition of metallic electrodes on a semiconductor medium is an indispensable factor in governing carrier injection, and a metal/semiconductor contact that can be formed via solution process is highly desired in printed electronics. However, fine‐patterning the solution processes of metallic electrodes without damaging the excellent electronic properties of organic semiconductors (OSCs) is still a challenge. In this work, electroless plating, a metal coating technique that involves auto‐catalytic reaction in an aqueous solution, is used to fabricate top‐contact organic thin‐film transistors (OTFTs). An electroless‐plated gold pattern with a spatial resolution of 10 micrometers is transferred and laminated on a monolayer of OSCs to serve as a hole‐injection electrode. The fabricated OTFTs exhibit reasonably high field‐effect mobility of up to 13 cm² V^?1 s^?1 and decent contact resistance as low as 120 Ω · cm, which implies that an ideal metal/semiconductor contact can be realized. This electroless plating technique can provide possibilities for practical mass production of organic integrated circuits because it is in principle cost‐effective, capable of covering large areas, high‐vacuum free, and environmentally friendly.     

Molecular Packing‐Induced Transition between Ambipolar and Unipolar Behavior in Dithiophene‐4,9‐dione‐Containing Organic Semiconductors

By changing the packing motif of the conjugated cores and the thin‐film microstructures, unipolar organic semiconductors may be converted into ambipolar materials. A combined experimental and theoretical investigation is conducted on the thin‐film organic field‐effect transistors (OFETs) of three organic semiconductors that have the same conjugated core structure of s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐4,9‐dione but with different n‐alkyl groups. The optical and electrochemical measurements suggest that the three organic semiconductors have very similar energy levels; however, their OFETs exhibit dramatically different transport characteristics. Transistors based on compound 1a or 1c show ambipolar transport properties, while those based on compound 1b show p‐type unipolar behavior. Specifically, compound 1c is characterized as a good ambipolar semiconductor with the highest electron mobility of 0.22 cm² V^?1 s^?1 and the highest hole mobility of 0.03 cm² V^?1 s^?1. Complementary metal oxide semiconductor (CMOS) inverters incorporated with compound 1c show sharp inversions with high gains above 50. Theoretical investigations reveal that the drastic difference in the transport properties of the three materials is due to the difference in their molecular packing and film microstructures.     

Alkyl‐Chain‐Length‐Independent Hole Mobility via Morphological Control with Poly(3‐alkylthiophene) Nanofibers

The field‐effect transistor (FET) and diode characteristics of poly(3‐alkylthiophene) (P3AT) nanofiber layers deposited from nanofiber dispersions are presented and compared with those of layers deposited from molecularly dissolved polymer solutions in chlorobenzene. The P3AT n‐alkyl‐side‐chain length was varied from 4 to 9 carbon atoms. The hole mobilities are correlated with the interface and bulk morphology of the layers as determined by UV–vis spectroscopy, transmission electron microscopy (TEM) with selected area electron diffraction (SAED), atomic force microscopy (AFM), and polarized carbon K‐edge near edge X‐ray absorption fine structure (NEXAFS) spectroscopy. The latter technique reveals the average polymer orientation in the accumulation region of the FET at the interface with the SiO₂ gate dielectric. The previously observed alkyl‐chain‐length‐dependence of the FET mobility in P3AT films results from differences in molecular ordering and orientation at the dielectric/semiconductor interface, and it is concluded that side‐chain length does not determine the intrinsic mobility of P3ATs, but rather the alkyl chain length of P3ATs influences FET diode mobility only through changes in interfacial bulk ordering in solution processed films.     

Tunable Crystal Nanostructures of Pentacene Thin Films on Gate Dielectrics Possessing Surface‐Order Control

To enhance the electrical performance of pentacene‐based field‐effect transistors (FETs) by tuning the surface‐induced ordering of pentacene crystals, we controlled the physical interactions at the semiconductor/gate dielectric (SiO₂) interface by inserting a hydrophobic self‐assembled monolayer (SAM, CH₃‐terminal) of organoalkyl‐silanes with an alkyl chain length of C8, C12, C16, or C18, as a complementary interlayer. We found that, depending on the physical structure of the dielectric surfaces, which was found to depend on the alkyl chain length of the SAM (ordered for C18 and disordered for C8), the pentacene nano‐layers in contact with the SAM could adopt two competing crystalline phases—a “thin‐film phase” and “bulk phase” – which affected the π‐conjugated nanostructures in the ultrathin and subsequently thick films. The field‐effect mobilities of the FET devices varied by more than a factor of 3 depending on the alkyl chain length of the SAM, reaching values as high as 0.6 cm² V^?1 s^?1 for the disordered SAM‐treated SiO₂ gate‐dielectric. This remarkable change in device performance can be explained by the production of well π‐conjugated and large crystal grains in the pentacene nanolayers formed on a disordered SAM surface. The enhanced electrical properties observed for systems with disordered SAMs can be attributed to the surfaces of these SAMs having fewer nucleation sites and a higher lateral diffusion rate of the first seeding pentacene molecules on the dielectric surfaces, due to the disordered and more mobile surface state of the short alkyl SAM.