Properties Of Low-dimensional Materials Research Articles

Structural and electronic properties of substitutionally doped SiAs monolayer

The substitutional single-atom doping is proven to be a significant method to alter the electronic properties of low dimensional materials. Based on the first-principles density functional theory, we present a systematic study on the structural and electronic properties of single-atom substitutionally doped SiAs monolayers. SiAs monolayer is a semiconductor with an indirect band gap of 1.6 eV. Through substitutional doping, the band gap (calculated at the PBE level) of the monolayer SiAs ranges from 0.032 eV to 1.625 eV. The Si31XAs32 (X = B, Al, Ga) and Si32As31Y (Y = C, Ge, Sn) systems present magnetic ground states with a magnetic moment of 1.00 μB. Moreover, monolayer Si31XAs32 (X = B, Al, Ga) and Si32As31Y (Y = C, Ge, Sn) have long range and local short range magnetic moments owing to the degree of hybridization between the dopant and its adjacent atoms. Metallic behavior can be found in Si31XAs32 (X = N, P, Sb), Si32As31Y (Y = O, S, Se) systems due to the contribution of doped atoms intersecting the Fermi level. The calculated results could provide a new approach for low dimensional doped SiAs-based photoelectronic and magnetic semiconductor devices.

An ab-initio perspective on the electronic and optical characteristics of MoSe2 nanosheet: Role of external electric field

The presence of an external electric field has a large influence on the properties of low-dimensional materials. Here, we examine the change of the band gap (Eg), the optical absorption coefficient (OAC), the refractive index (RI), and the optical conductivity of a monolayer MoSe2 (ML) with respect to the variation in the external electric field strength. Our results, based on a density-functional theory (DFT) calculation, exhibit an increase in the band gap when the electric field is turned on. However, we are witnessing a remarkable decrease in Eg for electric field strengths exceeding ±0.7VÅ−1. Further, we obtained a great correlation between the external electric field and the OAC. On the other hand, we have established that the light polarization perpendicular to the plane of the ML-MoSe2 shifts the position of the OAC peak to shorter wavelengths, and leads to a split of OAC peak located at 110nm. We have also achieved that the external electric field reduces the optical conductivity of the ML-MoSe2 in the UV region. That effect is found to be more pronounced when the MoSe2 nanosheet is subjected to a 001-polarized incident light and positive field strengths. However, the presence of an external electric field generally improves the optical conductivity in the visible region. Lastly, in the case of 001-polarization of the incident wave, we found that the RI shrinks in the UV region when the electric field is turned on. By contrast, the application of an external electric field tends to improve the refractive index in the visible region.

Giant Stark Effect in Two-Dimensional Hittorf’s Phosphorene

The electric field tunable band gap and optical properties in low-dimensional materials (quantum-confined Stark effect) are very useful in applications of optoelectronics. In this paper, based on the many-body perturbation method, we investigate the evolution of the quasiparticle electronic structure, exciton, and optical properties of two-dimensional (2D) Hittorf's phosphorene under an out-of-plane electric field. Compared to other 2D monolayers, the relatively large thickness of Hittorf's phosphorene leads to a significant reduction in the quasiparticle band gap when an electric field is applied along the quantum confinement direction. The unique bilayer structure, on the other hand, guarantees a well spatial separation of photon-excited electron-hole pairs and, consequently, reduced exciton binding energy under an out-of-plane electric field. These combined effects lead to an almost fixed exciton energy and optical absorption edge at low applied electric fields. However, when the field is larger than 0.1 V/\AA{}, substantial reductions in the exciton energy and optical absorption edge are identified. For the higher-order exciton states, the involvement of more complex band-to-band electron-hole pair formation results in a nonmonotonic electric field dependence. The effective optical modulation accompanied with these giant Stark effects shows potential applications of Hittorf's phosphorene in 2D optoelectronic devices.

Engineering of properties of low-dimensional materials via inhomogeneous strain

<sec>Low-dimensional material represents a special structure of matter. The exploring of its novel properties is an important frontier subject in the fundamental research of condensed matter physics and material science. Owing to its small length scale in one or two dimensions, low-dimensional materials are usually flexible in structure. This feature together with the prompt electronic response to structural deformations enable us to modulate the material properties via a strain way. The main purpose of this paper is to introduce the recent research progress of obtaining novel physical properties by inhomogeneously straining two-dimensional materials, with focusing on two effects, i.e., pseudomagnetic field effect and the flexoelectric effect. Of course, the influence of inhomogeneous strains on electrons is not limited to these two effects. Fundamentally, an inhomogeneous deformation breaks the symmetry of crystalline structure. This may serve as a start point to delineate the structural-properties relation. First, the symmetry breaking can eliminate the degeneracy of energy levels. Second, the symmetry breaking will also cause the heterogeneity of electronic and phonon properties in different parts of the material.</sec><sec>In the paper, we also introduce a special method named the generalized Bloch theorem that is suitable for dealing with the inhomogeneous strain patterns at an atomistic level. From the perspective of atomistic simulation, due to the breaking of translational symmetry, the standard quantum mechanical calculations encounter fundamental difficulties in dealing with an inhomogeneous strain, e.g., bending and torsion. The generalized Bloch method overcomes such an obstacle by considering rotational and/or screw symmetries given by bending and/or torsion in solving the eigenvalue problem. As such, quantum mechanical calculations can be still conducted with a relatively small number of atoms.</sec>

Research progress of surface atomic manipulation and physical property regulation of low-dimensional structures

Atomic manipulation technique with scanning tunneling microscopy (STM) has been used to control the structural and physical properties of materials at an atomic level. Recently, this technique has been extended to modifying the physical properties of low-dimensional materials. Unlike conventional single atom lateral manipulation, the STM manipulation technique in the study of low-dimensional materials has additional manipulation modes and focuses on the modification of physical properties. In this review paper, we introduce the recent experimental progress of tuning the physical properties of low-dimensional materials through STM atomic manipulation technique. There are mainly four manipulation modes: 1) tip-induced local electric field; 2) controlled tip approach or retract; 3) tip-induced non-destructive geometry manipulation; 4) tip-induced kirigami and lithography. Through using these manipulation modes, the STM tip effectively introduces the attractive force or repulsive force, local electronic field or magnetic field and local strain, which results in the atomically precise modification of physical properties including charge density wave, Kondo effect, inelastic tunneling effect, Majorana bound states, and edge states.

Numerical investigation of excitonic resonances of few-layer black phosphorus based on field enhancement

The optical absorption characteristic, which is one of the key parameters of two-dimensional black phosphorus when functioning as an optoelectronic semiconductor device, can be affected by the exciton binding energy. Field enhancement achieved by metal nanostructures can be used to adjust the photoelectric properties of low-dimensional materials. Here, we propose a new coupled field enhancement method by using metal nanorod arrays and Fabry–Perot cavity to improve the exciton-dominated optical absorption in few-layer black phosphorus. Our simulation results indicate that the exciton-dominated optical absorption can be improved to 84% (increased by 5.8-fold), while achieving a polarization-dependent optical absorption ratio of more than 30. Meanwhile, full width at half maximum of the field-enhanced formant and the polarization ratio can be improved by optimizing the geometric parameters of metal nanostructures. Our results reveal that coupled field enhancement of few-layer black phosphorus could provide a potential new scheme for tunable or polarization-sensitive optoelectronic devices.

Strain modulation using defects in two-dimensional MoS2

We investigate the nature of strain in $\mathrm{Mo}{\mathrm{S}}_{2}$ and correlate it to defect types and densities, while systematically assessing the tolerance of this low dimensional material to He and Au ion irradiations. Through a series of theoretical predictions and experimental observations, we establish the onset of the crystalline-to-amorphous transition in $\mathrm{Mo}{\mathrm{S}}_{2}$ and identify sulfur vacancies as the most favorable defects introduced during irradiation. We note the presence of both tensile and compressive strains, which depend on the types of defects introduced into the lattice and vary with increasing fluence. The results show that defects can be used to tune strain in two-dimensional materials and provide an exciting pathway for using external stimuli to control properties of low dimensional materials.

Impact of transverse and vertical gate electric field on vibrational and electronic properties of MoS2

Selectivity of the electric field direction plays a vital role in modulating the phonon characteristics as well as electrical properties in low-dimensional materials. A comprehensive study on the effects of the direction-dependent electric field on MoS2 sample is reported herewith. The field-induced changes in the phonon characteristics and electronic band structure have been systematically investigated based on field responsive Raman and photoluminescence measurements. The atomistic insights obtained from density functional theory calculations have been correlated with the experimental observations to elucidate the underlying mechanism. The applied transverse electric field is found to be significantly more efficacious than the electric field applied vertically in altering the phonon signatures and bandgap in MoS2, where the electrostrictive response is found to arise from the field-induced alteration in metal–chalcogen interatomic bonds.

Resonant tip-enhanced Raman scattering by CdSe nanocrystals on plasmonic substrates.

Tip-enhanced Raman scattering (TERS) has recently emerged as a powerful technique for studying the local properties of low dimensional materials. Being a plasmon driven system, a dramatic enhancement of the TERS sensitivity can be achieved by an appropriate choice of the plasmonic substrate in the so-called gap-mode configuration. Here, we investigate the phonon properties of CdSe nanocrystals (NCs) utilizing gap-mode TERS. Using the Langmuir–Blodgett technique, we homogeneously deposited submonolayers of colloidal CdSe NCs on two different nanostructured plasmonic substrates. Amplified by resonant gap-mode TERS, the scattering by the optical phonon modes of CdSe NCs is markedly enhanced making it possible to observe up to the third overtone of the LO mode reliably. The home-made plasmonic substrates and TERS tips allow the analysis of the TERS images of CdSe phonon modes with nanometer spatial resolution. The CdSe phonon scattering intensity is strongly correlated with the local electromagnetic field distribution across the plasmonic substrates.

(Invited) Thermoelectric properties of Fermi-level tuned and Aligned Single Wall Carbon Nanotubes

Single wall carbon nanotubes (SWCNTs) are one-dimensional materials with sharp van Hove singularities. Preparation of SWCNTs with a selected electronic structure, fabrication of their aligned thin films[1] and tuning their Fermi level allow us to reveal unique phenomena, which cannot be observed or correctly understood in conventional SWCNT random network thin films. For example, we observed anomalously large optical absorption for perpendicular polarization to the tube axis in highly doped aligned SWCNTs, which is related to intersubband transition plasmon phenomena in SWCNTs.[2] Here in this talk, we would like to discuss the thermoelectric phenomena in fermi level tuned and aligned SWCNTs. In our society, a large amount of heat at relatively low temperature, which is emitted from factories, houses, human bodies, and so on, is left unused. Development of high-performance flexible thermoelectric devices is crucial to efficiently convert such unused waste heat into electric power. Understanding of relationships between electrical conductivity, Seebeck coefficient, and thermal conductivity in thin films of flexible materials and tuning of these parameters is crucial to improve the performance. Since the seminal study by Hicks and Dresselhaus,[3] the thermoelectric properties of low dimensional materials have been intensively studied, but it still has been of great importance to experimentally clarify how the one dimensional electronic structures can influence and enhance its thermoelectric properties, although there are several important theoretical predictions for enhancement of thermoelectric properties in one-dimensional electronic systems.[3,4] SWCNTs are a model of one-dimensional flexible materials, and will play important roles for realization of high-performance flexible thermoelectric devices. Previously, we revealed how the location of Fermi-level influences the thermoelectric properties of semiconducting SWCNTs with diameter of 1.4 nm using electrolyte gating approaches.[5] However, in the previous study, the purity of chirality was not enough to clarify the intrinsic characteristics of semiconducting SWCNTs. In this talk, we will discuss the relationships between electronic structures, Fermi-level and thermoelectric properties using single chirality SWCNTs. In addition, we will discuss how the thermoelectric properties and conductivities exhibit isotropic or an-isotropic properties in aligned SWCNT thin films.[6] Acknowledgment: K.Y. acknowledges support by JST CREST through Grant Number JPMJCR17I5, Japan

Direct Reconstruction of Two-Dimensional Currents in Thin Films from Magnetic-Field Measurements

Accurate determination of microscopic transport and magnetization currents is of central importance for the study of the electric properties of low dimensional materials and interfaces, of superconducting thin films and of electronic devices. Current distribution is usually derived from the measurement of the perpendicular component of the magnetic field above the surface of the sample, followed by numerical inversion of the Biot-Savart law. The inversion is commonly obtained by deriving the current stream function $g$, which is then differentiated in order to obtain the current distribution. However, this two-step procedure requires filtering at each step and, as a result, oversmoothes the solution. To avoid this oversmoothing we develop a direct procedure for inversion of the magnetic field that avoids use of the stream function. This approach provides enhanced accuracy of current reconstruction over a wide range of noise levels. We further introduce a reflection procedure that allows for the reconstruction of currents that cross the boundaries of the measurement window. The effectiveness of our approach is demonstrated by several numerical examples.

Casting a Wider Net: Rational Synthesis Design of Low-Dimensional Bulk Materials.

The discovery of novel magnetic and electronic properties in low-dimensional materials has led to the pursuit of hierarchical materials with specific substructures. Low-dimensional solids are highly anisotropic by nature and show promise in new quantum materials leading to exotic physical properties not realized in three-dimensional materials. We have the opportunity to extend our synthetic strategy of the flux-growth method to designing single crystalline low-dimensional materials in bulk. The goal of this Account is to highlight the synthesis and physical properties of several low-dimensional intermetallic compounds containing specific structural motifs that are linked to desirable magnetic and electrical properties. We turned our efforts toward intermetallic compounds consisting of antimony nets because they are closely linked to properties such as high carrier mobility (the velocity of an electron moving through a material under a magnetic field) and large magnetoresistance (the change in resistivity with an applied magnetic field), both of which are desirable properties for technological applications. The SmSb2 structure type is of particular interest because it is comprised of rectangular antimony nets and rare earth ions stacked between the antimony nets in a square antiprismatic environment. LnSb2 (Ln = La-Nd, Sm) have been shown to be highly anisotropic with SmSb2 exhibiting magnetoresistance of over 50000% for H∥c axis and ∼2400% for H∥ab. Using this structure type as an initial building block, we envision the insertion of transition metal substructures into the SmSb2 structure type to produce ternary materials. We describe compounds adopting the HfCuSi2 structure type as an insertion of a tetrahedral transition metal-antimony subunit into the LnSb2 host structure. We studied LnNi1-xSb2 (Ln = Y, Gd-Er), where positive magnetoresistance reaching above 100% was found for the Y, Gd, and Ho analogues. We investigated the influence of the transition metal sublattice by substituting Ni into Ce(Cu1-xNix)ySb2 (y < 0.8) and found that the material is highly anisotropic and metamagnetic transitions appear at ∼0.5 and 1 T in compounds with higher Ni concentration. Metamagnetism is characterized by a sharp increase in the magnetic response of a material with increasing applied magnetic field, which was also observed in LnSb2 (Ln = Ce-Nd). We also endeavored to study materials that possess a transition metal sublattice with the potential for geometric frustration. An example is the La2Fe4Sb5 structure type, which consists of antimony square nets and an iron-based network arranged in nearly equilateral triangles, a feature found in magnetically frustrated systems. We discovered spin glass behavior in Ln2Fe4Sb5 (Ln = La-Nd, Sm) and evidence that the transition metal sublattice contributes to the magnetic interactions of Ln2Fe4Sb5. We investigated the magnetic properties of Pr2Fe4-xCoxSb5 (x < 2.3) and found that as the Co concentration increases, a second magnetic transition leads from a localized to an itinerant system. The La2Fe4Sb5 structure type is quite robust and allows for the incorporation of other transition metals, thereby making it an excellent candidate to study competing magnetic interactions in lanthanide-containing intermetallic compounds. In this manuscript, we aim to share our experiences of bulk intermetallic compounds to inspire the development of new low-dimensional materials.

First-Principles Simulation on Thermoelectric Properties in Low-Dimensional Materials

Thermoelectric properties were simulated for low-dimensional atomistic model structures based on first-principles calculation. New methodology about the first-principles simulation on Seebeck coefficient at arbitrary temperature and carrier concentration is presented. Dependence of Seebeck coefficient on scale, temperature, and carrier concentration has been demonstrated for silicon and beta silicon carbide nanowire models. Compared with the corresponding bulk models, a significant increase of the absolute value of Seebeck coefficient can be observed owing to quantum confinement by dimensional reduction. By the simulation with considering the energy dependence of the relaxation time, the Seebeck coefficient from the viewpoint of first principles can be evaluated as a range determined by the scattering constants peculiar to particular scattering processes.

Tuning Surface Properties of Low Dimensional Materials via Strain Engineering.

The promising and versatile applications of low dimensional materials are largely due to their surface properties, which along with their underlying electronic structures have been well studied. However, these materials may not be directly useful for applications requiring properties other than their natal ones. In recent years, strain has been shown to be an additionally useful handle to tune the physical and chemical properties of materials by changing their geometric and electronic structures. The strategies for producing strain are summarized. Then, the electronic structure of quasi-two dimensional layered non-metallic materials (e.g., graphene, MX2, BP, Ge nanosheets) under strain are discussed. Later, the strain effects on catalytic properties of metal-catalyst loaded with strain are focused on. Both experimental and computational perspectives for dealing with strained systems are covered. Finally, an outlook on engineering surface properties utilizing strain is provided.

Metallic and ferromagnetic MoS2 nanobelts with vertically aligned edges

Edge effects are predicted to significantly impact the properties of low dimensional materials with layered structures. The synthesis of low dimensional materials with copious edges is desired for exploring the effects of edges on the band structure and properties of these materials. Here we developed an approach for synthesizing MoS2 nanobelts terminated with vertically aligned edges by sulfurizing hydrothermally synthesized MoO3 nanobelts in the gas phase through a kinetically driven process; we then investigated the electrical and magnetic properties of these metastable materials. These edge-terminated MoS2 nanobelts were found to be metallic and ferromagnetic, and thus dramatically different from the semiconducting and nonmagnetic two-dimensional (2D) and three-dimensional (3D) 2H-MoS2 materials. The transitions in electrical and magnetic properties elucidate the fact that edges can tune the properties of low dimensional materials. The unique structure and properties of this one-dimensional (1D) MoS2 material will enable its applications in electronics, spintronics, and catalysis.

Recent Advances in the Development of Low-Dimensional Materials for Electrochemical Applications: CECRI, Karaikudi

Low-dimensional nanomaterials with exciting novel properties and magical performance is a rapidly growing research field that is truly multidisciplinary. A great deal of research efforts has been dedicated in the past two decades towards the preparation and characterization of these low-dimensional nanomaterials. The literature blossomed on nanomaterials already indicated about the origin of novel properties with size. However, it is equally important to know how the properties of materials and their function vary with their dimension without any size effects? To unravel this interesting question, researchers at CSIRCECRI were working towards the development of low-dimensional materials such as 1-D and 2-D nanomaterials and study their interesting properties with special emphasis on electrochemical applications. Recent research findings from our institute suggest that the tunable properties of these low-dimensional materials are attractive for a variety of applications such as energy storage, sensing, catalysis and in optoelectronics.

Size Distribution and Frustrated Antiferromagnetic Coupling Effects on the Magnetic Behavior of Ultrafine Akaganéite (β-FeOOH) Nanoparticles

The magnetic properties of low dimensional materials of several iron oxyhydroxide phases, such as akaganeite (β-FeOOH) or lepidocrocite (γ-FeO(OH)), remain poorly explored, probably due to their sp...

Tuning electronic and magnetic properties of AlN nanosheets with hydrogen and fluorine: First-principles prediction

We performed first-principles calculations to study the electronic structures and magnetic properties of the two-dimensional AlN nanosheet decorated with hydrogen and fluorine. The results show that the band gap of AlN nanosheet can be tuned significantly, and they can be a direct or an indirect semiconductor when decorated with H or F atoms on AlN surface. Spin-polarized calculations show that semi-decorated AlN sheets with H or F atoms can present a half-metal or p-type ferromagnetic (FM) semiconductor with Curie temperatures above room temperature. More interestingly, when AlN nanosheet co-decorated with H and F on each side, they show anisotropic semiconducting characters with a band gap of 1.02 eV. Our studies demonstrate that the decoration III–V group semiconductor nanosheets with foreign atoms might be an efficient route to tune the electronic and magnetic properties in low-dimensional materials.

ChemInform Abstract: Determination of the Local Structure and Electronic States of High-Tc Superconductors by Scanning Tunneling Microscopy

To date, scanning tunneling microscopy (STM) has significantly advanced the understanding of the local structure, electronic properties, and reactivity of semiconductor surfaces. Less recognized is the fact that STM can also elucidate the local structural and electronic properties of low-dimensional materials. Herein, the authors demonstrate that STM can provide new and essential insight into the physical complexities of high-temperature copper oxide superconductors that is unavailable from conventional studies. After reviewing the basic theoretical concepts needed to evaluate STM data, the authors discuss several specific cases that illustrate the unique information that STM provides. These examples include (1) the atomic level nature of structural disorder in the BiO and TlO layers of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} and Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8} and the low-energy electronic states associated with these structural features, (2) the local structure and electronic consequences of metal substitution and oxygen doping in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}, and (3) low-temperature tunneling spectroscopy measurements of the superconducting energy gap. Lastly, the authors summarize exciting future avenues to pursue in STM studies of these and other materials. 57 refs., 15 figs.

Size Effect on Thermal Properties in Low-Dimensional Materials

An extension of the classical thermodynamics to nanometer scale has been conducted to elucidate information regarding size dependence of phase transition functions and binary phase diagrams. The theoretical basis of the extension is Lindemann′s criterion for solid melting, Mott′s expression for vibrational melting entropy, and Shi′s model for size dependent melting temperature. These models are combined into a unified one without adjustable parameters for melting temperatures of nanocrystals. It is shown that the melting temperature of nanocrystals may drop or rise depending on interface conditions and dimensions. The model has been extended and applied to size dependences of melting enthalpy, melting entropy, atomic cohesive energy. Moreover, the above modeling has been utilized to determine the size-dependent continuous binary solution phase diagrams. These thermodynamic approachs have extended the capability of the classical thermodynamics to the thermodynamic phenomena in the nanometer regime.