Modification Of Electronic Properties Research Articles

Strain Modulation of Electronic Properties of Monolayer Black Phosphorus

Recent advances in the fabrication of monolayer black phosphorus (MBP) call for a detailed understanding of the physics underlying the electronic structure and related modulation by the method of strain engineering. Here, we present an analytic study to explore the uniaxial strain effect of band structure in MBP based on the first-principles calculations and atomic-bond-relaxation correlation mechanism. It was found that the stress responses of MBP show evident anisotropy due to different edge type structures. The electronic band structure of MBP can be tuned by the applied strain. Moreover, we propose an analytic expression for the variation of the bandgap induced by the uniaxial strain from the perspective of atomistic origin, which suggests an effective bridge between the measurable quantities and the atomic bond identities of MBP. The underlying mechanism on the strain-dependent band offset can be attributed to the variation of crystal potential induced by the changes of bond length, strength, and ang...

Temperature-Triggered Sulfur Vacancy Evolution in Monolayer MoS2 /Graphene Heterostructures.

The existence of defects in 2D semiconductors has been predicted to generate unique physical properties and markedly influence their electronic and optoelectronic properties. In this work, it is found that the monolayer MoS2 prepared by chemical vapor deposition is nearly defect-free after annealing under ultrahigh vacuum conditions at ≈400 K, as evidenced by scanning tunneling microscopy observations. However, after thermal annealing process at ≈900 K, the existence of dominant single sulfur vacancies and relatively rare vacancy chains (2S, 3S, and 4S) is convinced in monolayer MoS2 as-grown on Au foils. Of particular significance is the revelation that the versatile vacancies can modulate the band structure of the monolayer MoS2 , leading to a decrease of the bandgap and an obvious n-doping effect. These results are confirmed by scanning tunneling spectroscopy data as well as first-principles theoretical simulations of the related morphologies and the electronic properties of the various defect types. Briefly, this work should pave a novel route for defect engineering and hence the electronic property modulation of three-atom-thin 2D layered semiconductors.

Evolution of Landau levels in graphene-based topological insulators in the presence of wedge disclinations

In this paper we consider modification of electronic properties of graphene-based topological insulator in the presence of wedge disclination and magnetic field by adopting the Kane–Mele model with intrinsic spin–orbit coupling. Using the properly defined Dirac–Weyl equation for this system, an exact solution for the Landau levels is obtained. The influence of the topological defect on the evolution of Landau levels is discussed.

Ambipolar transport in Mn2CoAl films by ionic liquid gating

We demonstrate ambipolar transport and modulation of electronic properties of Mn2CoAl (MCA), one of the most promising candidates for spin gapless semiconductors (SGSs), by using ionic liquid gating in electronic double-layer transistors. The carrier concentration and mobility of MCA films were systematically changed with carrier polarity inversion from the p- to the n-type by the gating technique. The ambipolar transport is one of the most significant properties of SGSs and strongly promotes the gapless features of MCA. The present results pave the way for the use of MCA as a spin source of both spin-polarized electrons and holes.

Irreparable Defects Produced by the Patching of h-BN Frontiers on Strongly Interacting Re(0001) and Their Electronic Properties.

Clarifying the origin and the electronic properties of defects in materials is crucial since the mechanical, electronic and magnetic properties can be tuned by defects. Herein, we find that, for the growth of h-BN monolayer on Re(0001), the patching frontiers of different domains can be classified into three types, i.e., the patching of B- and N-terminated (B|N-terminated) frontiers, B|B-terminated frontiers and N|N-terminated frontiers, which introduce three types of defects, i.e., the "heart" shaped moiré-level defect, the nonbonded and bonded line defects, respectively. These defects were found to bring significant modulations to the electronic properties of h-BN, by introducing band gap reductions and in-gap states, comparing with perfect h-BN on Re(0001) with a band gap of ∼3.7 eV. The intrinsic binary composition nature of h-BN and the strong h-BN-Re(0001) interaction are proposed to be cooperatively responsible for the formation of these three types of defects. The former one provides different types of h-BN frontiers for domain patching. And the later one induces multinucleation but aligned growth of h-BN domains on Re(0001), thus precluding their subsequent coalescence to some extent. This work offers a deep insight into the categories of defects introduced from the patching growth of two-dimensional layered materials, as well as their electronic property modulation through the defect engineering.

Architecture of 3D ZnCo2O4 marigold flowers: Influence of annealing on cold emission and photocatalytic behavior

The present work demonstrates the field emission characteristics and photocatalytic behavior of ZnCo2O4 marigold flowers synthesized via a facile hydrothermal method. The effect of annealing of these 3D porous hierarchical nanostructures on field emission and photocatalytic performances is studied. When compared with the as-synthesized sample, annealed ZnCo2O4 exhibits a ∼2-fold improvement in photocatalytic activity for methylene blue (MB) degradation under visible light irradiation. The turn-on, threshold fields and high emission current densities are also strongly influenced as a result of annealing. The turn on field required to draw an emission current density of ∼1 μA/cm2 is found to be ∼2.4 and ∼1.8 V/μm for as-synthesized and annealed ZnCo2O4 marigold flowers, respectively. Field-emission measurements demonstrate remarkably large field enhancement and better stability for annealed samples. The X-ray diffraction and Raman analysis reveal that annealing improves the crystallinity and also help to remove the structural defects in ZnCo2O4. The enhancement in the field emission and photocatalytic activity of annealed ZnCo2O4 marigold flowers is attributed to the modification of electronic properties as a result of dehydration, crystallite growth and reduced surface defects/impurity phases.

Enhanced Field Emission Characteristics of a 3D Hierarchical HfO2‐ZnO Heteroarchitecture

AbstractThree dimensional (3D) HfO2‐ZnO heteroarchitecture comprised of thin coating of HfO2 on self assembled 3D ZnO urchins with pointed apex has been synthesized using hydrothermal route followed by Pulsed Laser Deposition (PLD). The as‐synthesized HfO2‐ZnO heteroarchitecture was characterized using XRD, SEM, EDS, and (HR) TEM, in order to reveal its structural, morphological, and chemical properties. The HfO2‐ZnO heteroarchitecture emitter exhibits superior field emission (FE) behaviour in contrast to the pristine ZnO urchins, demonstrated by delivery of high emission current density of ∼ 885 μA/cm2 at an applied field of ∼ 3.35 V/μm, against ∼383 μA/cm2 at an applied field of ∼ 4.32 V/μm for the pristine ZnO urchins emitter. Interestingly, the HfO2‐ZnO heteroarchitecture emitter exhibits excellent emission current stability characterized with fewer fluctuations, owing to very good ion‐bombardment resistance offered by the HfO2 coating. Furthermore, the heteroarchitecture thus obtained facilitates tailoring of the morphology with high aspect ratio and modulation of electronic properties as well, thereby enhancing the FE behaviour.Despite HfO2 being wide band gap and high‐k material, the HfO2‐ZnO heteroarchitecture exhibits potential as promising candidate for fabrication of high current density cold cathode

One step hydrothermal synthesis of SnO2-RGO nanocomposite and its field emission studies

Field emission (FE) properties of hydrothermally synthesized, SnO2-RGO nanocomposite have been investigated at a base pressure of 1×10−8mbar. The results reveal that the SnO2-RGO nanocomposite emitter prevails over the pristine RGO emitter. The values of turn-on field, defined at emission current density of 1 μA/cm2, are found to be 1.8 and 2.2V/μm for the SnO2-RGO and pristine RGO emitters, respectively. Furthermore, the SnO2-RGO emitter delivers maximum emission current density of ~800µA/cm2 at an applied field of 5V/μm. The observed values of applied field corresponding to emission current densities of 1 μA/cm2 and 10µA/cm2 are superior to those reported for various emitters due to SnO2 nanostructures and their composites. The emission current at the pre-set value of 1µA is found to be very stable over a period of 3hrs. The enhanced FE behaviour of SnO2-RGO nanocomposite emitter has been attributed to synergic effect due to its nanometric dimensions offering high aspect ratio and modulation of electronic properties via formation of heterostructure. The results obtained herein propose the SnO2-RGO nanocomposite as a prospective candidate for FE based vacuum microelectronic devices.

Strain and water effects on the electronic structure and chemical activity of in-plane graphene/silicene heterostructure

By using first-principles calculations, the electronic structure of planar and strained in-plane graphene/silicene heterostructure is studied. The heterostructure is found to be metallic in a strain range from −7% (compression) to +7% (tension). The effect of compressive/tensile strain on the chemical activity of the in-plane graphene/silicene heterostructure is examined by studying its interaction with the H2O molecule. It shows that compressive/tensile strain is able to increase the binding energy of H2O compared with the adsorption on a planar surface, and the charge transfer between the water molecule and the graphene/silicene sheet can be modulated by strain. Moreover, the presence of the boron-nitride (BN)-substrate significantly influences the chemical activity of the graphene/silicene heterostructure upon its interaction with the H2O molecule and may cause an increase/decrease of the charge transfer between the H2O molecule and the heterostructure. These findings provide insights into the modulation of electronic properties of the in-plane free-standing/substrate-supported graphene/silicene heterostructure, and render possible ways to control its electronic structure, carrier density and redox characteristics, which may be useful for its potential applications in nanoelectronics and gas sensors.

Modulation of electronic properties of tin oxide nanobelts via thermal control of surface oxygen defects

Nanomaterials made from binary metal oxides are of increasing interest because of their versatility in applications from flexible electronics to portable chemical and biological sensors. Controlling the electrical properties of these materials is the first step in device implementation. Tin dioxide (SnO2) nanobelts (NB) synthesized by the vapor–liquid–solid mechanism have shown much promise in this regard. We explore the modification of devices prepared with single crystalline NBs by thermal annealing in vacuum and oxygen, resulting in a viable field-effect transistor (FET) for numerous applications at ambient temperature. An oxygen annealing step initially increases the device conductance by up to a factor of 105, likely through the modification of the surface defects of the NB, leading to Schottky barrier limited devices. A multi-step annealing procedure leads to further increase of the conductance by approximately 350% and optimization of the electronic properties. The effects of each step is investigated systematically on a single NB. The optimization of the electrical properties of the NBs makes possible the consistent production of channel-limited FETs and control of the device performance. Understanding these improvements on the electrical properties over the as-grown materials provides a pathway to enhance and tailor the functionalities of tin oxide nanostructures for a wide variety of optical, electronic, optoelectronic, and sensing applications that operate at room temperature.

Influence of laser irradiation on local electronic properties of graphene in the presence of water adsorbate

The effect of volatility of local electronic properties in multilayered (3–6 layers) graphene irradiated by nanosecond laser pulses with a wavelength 532nm in humid atmosphere was found. The experiments were carried out in situ in a scanning probe microscope (SPM) with Pt/Si tip. It was established that along with local profile transformation of graphene sheet (the formation of "nanocrater" with depth up to 2nm and ~1µm diameter) in the zone of the laser action (~0,5µm) a decrease (~20meV) of the electrons work function was observed. It takes place in the region larger but comparable with the irradiation spot size. Analysis of the experimental data on local electrical conductivity have shown that the bottom of conduction band for graphene in laser irradiation area was reduced by ~ 50meV. The observed changes of graphene physical properties were associated with the laser-induced radial displacement of the water layer intercalated between graphene and substrate. In addition, it was experimentally established that the graphene work function was decreased significantly less than can be expected for an "ideal" 3 - 6 layer graphene. The investigated process of laser-induced changes of graphene properties demonstrated the reproducible character and long-term stability. Possibility of laser fabrication of arrays in graphene with periodic modulation of electronic properties was demonstrated.

Modulation of electrical potential and conductivity in an atomic-layer semiconductor heterojunction.

Semiconductor heterojunction interfaces have been an important topic, both in modern solid state physics and in electronics and optoelectronics applications. Recently, the heterojunctions of atomically-thin transition metal dichalcogenides (TMDCs) are expected to realize one-dimensional (1D) electronic systems at their heterointerfaces due to their tunable electronic properties. Herein, we report unique conductivity enhancement and electrical potential modulation of heterojunction interfaces based on TMDC bilayers consisted of MoS2 and WS2. Scanning tunneling microscopy/spectroscopy analyses showed the formation of 1D confining potential (potential barrier) in the valence (conduction) band, as well as bandgap narrowing around the heterointerface. The modulation of electronic properties were also probed as the increase of current in conducting atomic force microscopy. Notably, the observed band bending can be explained by the presence of 1D fixed charges around the heterointerface. The present findings indicate that the atomic layer heterojunctions provide a novel approach to realizing tunable 1D electrical potential for embedded quantum wires and ultrashort barriers of electrical transport.

First-principles study of size-, surface- and mechanical strain-dependent electronic properties of wurtzite and zinc-blende InSb nanowires

Using first-principle calculations with density functional theory, we investigated the modification of electronic properties in zinc-blende (ZB) and wurtzite (WZ) InSb nanowires (NWs) grown along the [111] and [0001] directions for different size, different surface coverage and different mechanical strain. The results show that before the surface passivation, ZBNWs and WZNWs exhibit the metallic character and the semiconductor character, respectively. WZNWs show a crossover from a direct to an indirect as diameter decreases. After the surface passivation, both ZBNWs and WZNWs are found to be direct-gap character. The electronic band structure shows a significant response to changes in surface passivation with pseudo hydrogen and halogen. The band structure with mechanical strain is strongly dependent on the crystal orientation and the NW diameter. In ZBNWs, compressive strain induces the indirect band gap character, whereas tensile strain can not form it. WZNWs have various strain dependence in that both compressive and tensile strain make InSb show a direct band gap character. A brief analysis of these results is given.

Modification of electronic properties of graphene by using low-energy K+ ions

Despite its superb electronic properties, the semi-metallic nature of graphene with no band gap (Eg) at the Dirac point has been a stumbling block for its industrial application. We report an improved means of producing a tunable band gap over other schemes by doping low energy (10 eV) potassium ions (K+) on single layer graphene formed on 6H-SiC(0001) surface, where the noble Dirac nature of the π-band remains almost unaltered. The changes in the π-band induced by K+ ions reveal that the band gap increases gradually with increasing dose (θ) of the ions up to Eg = 0.65 eV at θ = 1.10 monolayers, demonstrating the tunable character of the band gap. Our core level data for C 1s, Si 2p, and K 2p suggest that the K+-induced asymmetry in charge distribution among carbon atoms drives the opening of band gap, which is in sharp contrast with no band gap when neutral K atoms are adsorbed on graphene. This tunable K+-induced band gap in graphene illustrates its potential application in graphene-based nano-electronics.

Modulation of electronic properties from stacking orders and spin-orbit coupling for 3R-type MoS2

Two-dimensional crystals stacked by van der Waals coupling, such as twisted graphene and coupled graphene-BN layers with unusual phenomena have been a focus of research recently. As a typical representative, with the modulation of structural symmetry, stacking orders and spin-orbit coupling, transitional metal dichalcogenides have shown a lot of fascinating properties. Here we reveal the effect of stacking orders with spin-orbit coupling on the electronic properties of few-layer 3R-type MoS2 by first principles methods. We analyze the splitting of states at the top of valence band and the bottom of conduction band, following the change of stacking order. We find that regardless of stacking orders and layers’ number, the spin-up and spin-down channels are evidently separated and can be as a basis for the valley dependent spin polarization. With a model Hamiltonian about the layer’s coupling, the band splitting can be effectively analyzed by the coupling parameters. It is found that the stacking sequences, such as abc and abca, have the stronger nearest-neighbor coupling which imply the popular of periodic abc stacking sequence in natural growth of MoS2.

Fluorodiphenylethene-Containing Donor–Acceptor Conjugated Copolymers with Noncovalent Conformational Locks for Efficient Polymer Field-Effect Transistors

The diphenylethene moiety is a versatile building block that offers several chemically functionalizable sites, allowing easy modulation of electronic properties of the resulting polymers and providing numerous opportunities for discovering related structure–property relationships. In this study, we report a series of difluorodiphenylethene-based copolymers with noncovalent conformational locks for applications in polymer field-effect transistors. Different fluorination positions lead to different type of intra- and intermolecular interactions, backbone conformations, and eventually different device performances. 2,2′-Difluorodiphenylethene-based copolymers P2DFPE-n containing F···H–C conformation locks exhibit obviously enhanced hole mobilities of 1.3–1.5 cm2 V–1 s–1, whereas 3,3′-difluorodiphenylethene-based copolymers P3DFPE-n containing F···H–C and F···S conformation locks show lower mobilities of 0.2–0.4 cm2 V–1 s–1. AFM and 2D-GRXD investigations indicate that P2DFPE-n takes predominantly edge-on ori...

Effects of strain on electronic properties of monolayer α-Fe2O3

Density functional theory was employed to study the strain-induced modification of electronic properties of advanced monolayer α-Fe2O3 for the first time. Theoretical results indicated that there was obvious dependency of band structure on strain. The band gap modulation could reach up to −29.0% for 7% compressive strain and to 10.6% for 7% tensile strain. The analytical results of fat-band structures demonstrated that the considerable modulation range of band gaps was mainly caused by the Fe-dz2 orbital.

Facile, single step synthesis of CdS—RGO heterostructure and its photo enhanced field emission investigations

CdS–RGO heterostructure was synthesized by a thermal evaporation method and characterized using x-ray diffraction, SEM, TEM, photoluminescence spectroscopy (PL) and Raman spectroscopy so as to reveal its structural, compositional and electronic properties, prior to field emission (FE) investigations. The FE characteristics with and without visible illumination of the CdS–RGO heterostructure planar emitter were measured at base pressure 1 × 10−8 mbar. The values of turn-on field, corresponding to emission current density of 1 μA/cm2 without and with illumination, are found to be 2.3 and 2.0 V μm−1, respectively. Furthermore, under pulsating illumination, the emitter exhibits emission current pulses depicting nearly four times enhancement in the emission current, with rise and fall time constants as 4.4 and 6.4 s, respectively. The photoenhanced FE behaviour is attributed to the modulation of electronic properties due to the CdS nanostructures. The results obtained herein propose the CdS–RGO heterostructure emitter as a photo-sensitive field emission switch in next generation optoelectronic nanodevices.

Efficient nitrogen doping of graphene by plasma treatment

Doping of pristine materials can change their chemical and electrical properties. Namely nitrogen doping of graphene results in modulation of electronic properties of graphene. In this work we present experimental results on nitrogen doped graphene fabricated in two steps. At first, the graphene samples were synthesized by a chemical vapor deposition method on copper foils. Then they were treated with ammonia radio frequency discharge plasma. The prepared samples were investigated by atomic-force microscopy (AFM), scanning electron microscopy (SEM), Raman spectroscopy, optical absorption spectroscopy including Fourier transform infrared spectroscopy (FTIR), X-ray and ultraviolet photoelectron spectroscopy. In doped graphene a dependence of N-atom concentration on the treatment parameters has been revealed. A maximum doping level of 3 atomic % has been obtained and the shift of valence band maximum of 0.2 eV was observed at this concentration of nitrogen.

Electronic Property Modulation of One-Dimensional Extended Graphdiyne Nanowires from a First-Principle Crystal Orbital View.

Graphdiyne and derivatives with delocalized π‐electron systems are of particular interest owing to their structural, electronic, and transport properties, which are important for potential applications in next‐generation electronics. Inspired by recently obtained extended graphdiyne nanowires, explorations of the modulation of the band gap and carrier mobility of this new species are still needed before application in device fabrication. To provide a deeper understanding of these issues, herein we present theoretical studies of one‐dimensional extended graphdiyne nanowires using first‐principle calculations. Modulation of the electronic properties of the extended graphdiyne nanowire was investigated systemically by considering several chemical and physical factors, including electric field, chemical functionalization, and carbo‐merization. The band gap was observed to increase upon application of an electric field parallel to the plane of the synthesized graphdiyne nanowire in a non‐periodic direction. Although chemical functionalization and carbo‐merization caused the band gaps to decrease, the semiconducting property of the nanowires was preserved. Band gap engineering of the extended graphdiyne nanowires was explored regarding the field strength and the number of −C≡C− units in the carbon chain fragments. The charge carrier mobility of chemically functionalized and carbo‐merized extended graphdiyne nanowires was also calculated to provide a comparison with pristine nanowire. Moreover, crystal orbital analysis was performed in order to discern the electronic and charge transport properties of the extended graphdiyne nanowires modified by the aforementioned chemical and physical factors.