Low-dimensional Nanostructures Research Articles

A comprehensive tutorial on the pulsed laser deposition technique and developments in the fabrication of low dimensional systems and nanostructures

A comprehensive tutorial on the pulsed laser deposition technique and developments in the fabrication of low dimensional systems and nanostructures

Endohedral group-14-element clusters TM@E9 (TM = Co, Ni, Cu; E = Ge, Sn, Pb) and their low-dimensional nanostructures: a first-principles study.

Endohedral group14-based clusters with the encapsulation of a transition metal, which are termed [TM@Em]n- (TM = transition metal and E = group-14 elements), have lots of potential applications and have been used as interesting building blocks in materials science. Nevertheless, their electronic structures and stability mechanism remain unclear. In this paper, we systematically study the geometries, electronic structures, and bonding properties of [TM@E9]n- clusters which are the smallest endohedral group-14-based clusters synthesized so far, by using density functional theory (DFT) calculations. The calculation results reveal the important role of TMs in affecting the structures and bonding interactions in the [TM@E9]n- cluster. In the presence of a TM, the cluster geometry could change from a monocapped square antiprism (C4v) for empty [E9]4- cages to a tricapped trigonal prismatic geometry (D3h) for [TM@E9]n-. By using the energy decomposition analysis (EDA) method, the bonding properties between the endohedral TM and E9 cluster have been thoroughly investigated. It was found that the origin of stability of these clusters is from the large electrostatic attraction with significantly reduced Pauli repulsion. In the case of orbital interactions, the π back-donations from d orbitals of the TM to the cluster make important contributions. More interestingly, the 1D-chain and 2D-sheet nanostructures based on the [Ni@E9] cluster have been theoretically predicted. The band structure and density of states analysis revealed that all of these nanostructures are metallic and their excellent thermodynamic stability has been confirmed by using ab initio molecular dynamics (AIMD) simulations.

Formation mechanism of InAs nanostructures on GaAs (001) surface at low temperature

In recent years, low-dimensional nanostructures such as quantum dots (QD) and quantum rings (QR) have been widely used in many fields such as optoelectronics, microelectronics and quantum communication due to their unique electrical, optical and magnetic properties. Owing to the similarity between nanostructures and atomic systems, the flexible modulation of several quantum properties of nanomaterials and the preparation of new optoelectronic devices around the characteristics of these structural systems have become a hot topic of research. Changing the growth process to control and tune the atomic diffusion mechanism in droplets is a key way of preparing complex nanostructures, which is important for the study of semiconductor nanostructure by droplet epitaxy. In the present experiment, the same amount (5 monolayer (5 ML)) of indium is deposited on GaAs (001) at different substrate temperatures (140, 160, 170 and 180 ℃) and different arsenic pressures (1.6, 3.3 and 4.6 ML/s), and the surface morphology evolutions are observed. As the substrate temperature increases, the radius of the disk gradually expands and a pit appears in the center of the diffusion disk. As the arsenic pressure increases, the density of the formed droplets increases, and the width of the diffusion disk formed in the center of the droplets gradually decreases. Our work involving nucleation theory is done at <i>T</i> < 200 ℃ to deactivate many thermal processes. This is a result of the diffusion coefficient being more complexly related to temperature. Based on the classical nucleation diffusion theory, the results of experimental data fitting include that the diffusion activation energies of In atoms on the surface of GaAs (001) are (0.62 ± 0.01) eV in <inline-formula><tex-math id="Z-20210928040627-1">\begin{document}$ [1\bar 10] $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20210645_Z-20210928040627-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="19-20210645_Z-20210928040627-1.png"/></alternatives></inline-formula>and (1.37 ± 0.01) eV in [110] respectively, and that the diffusion coefficient <i>D</i><sub>0</sub> is 1.2 × 10<sup>–2</sup> cm<sup>2</sup>/s:those results confirm the theory after having been compared with the results obtained by other research groups. The diffusion activation energy of indium atoms and the diffusion mechanism of indium droplets on GaAs (001) obtained from the experiment can provide experimental guidance for modulating the structural property of InAs nanostructures.

Anomalous thermal conductivity enhancement in low dimensional resonant nanostructures due to imperfections.

Nanophononic metamaterials have broad applications in fields such as heat management, thermoelectric energy conversion, and nanoelectronics. Phonon resonance in pillared low-dimensional structures has been suggested to be a feasible approach to reduce thermal conductivity (TC). In this work, we study the effects of imperfections in pillared nanostructures based on graphene nanoribbons (GNR), using classical molecular dynamics simulations and harmonic lattice dynamics. The TC of perfect pillared GNR is only about 13% of that of pristine GNR due to the strong phonon resonant hybridization in pillared GNR. However, introducing imperfections such as vacancy defects and mass mismatch between the pillars and the base material, and alloy disorder in the pillars, can weaken the resonant hybridization and abnormally increase the TC. We show that both vacancy defects and mass mismatch can reduce the penetration of the resonant modes from the pillars into the base material, while the alloy disorder in the pillars can scatter the phonons inside them, which turns regular resonance into a random one with weaker hybridization. Our work provides useful insight into the phonon resonance mechanisms in experimentally relevant low dimensional nanostructures containing various imperfections.

Electrodeposition of Metal Chalcogenides Nanostructures for Thermoelectrics

Metal Chalcogenide nanostructures have been extensively implemented for high-performance thermoelectric applications, which can directly convert waste-heat-energy into electricity, with the support of theoretical and empirical results. In fact, finding a novel and cost effective synthesis approach that facilitates the formation of stable and reproducible nanostructured thermoelectric materials determines the commercial applicability. The overall objective of this work is to synthesize metal chalcogenide nanostructures with outstanding thermoelectric performance by an electrodeposition technique, which is one of the most versatile methods for low dimensional nanostructures in a cost effective and scalable manner. Precise control over dimension, chemical composition, crystallinity, crystal structure, grain size, preferred orientation, and the attainment of nanoinclusions and/or intermediate phases based on thermodynamically favored solid-state phase transition yields optimal electrical transport and thermoelectric properties of the metal chalcogenide nanostructures. Herein, the electrodeposited nanostructures represented the enhancement of Seebeck coefficient without reducing the conductivity was investigated that was explained by a carrier energy filtering effect due to the band bending at the two-phase interfaces.

Influence of oxygen-related surface adsorbates on the growth of low dimensional nanostructures and enhanced luminescence due to superoxide charge-transfer states in ZnO for application in optoelectronic devices

Kinetically controlled growth of low dimensional zinc oxide (ZnO) nanostructures like nanorods, nanoflowers etc. have been demonstrated with enhanced photoluminescent emission by tuning of the hydroxide ion concentration in the precursor irrespective of the synthesis techniques. Change in concentration of the hydroxide ions in the precursor during the synthesis of ZnO nanostructured samples has been found to tune the aspect ratio of the nanostructures, viz. nanorods grown by electrodeposition, and morphology of nanoflowers to nanogranules synthesized by direct precipitation method. The photoluminescent emission from these samples has been found to increase synchronously with the increase in the concentration of hydroxide ions in the precursor. Observed enhancement in photoluminescence has been proposed to be due to the formation of superoxide (O2−) charge transfer (CT) states CTO2− due to the adsorption of molecular oxygen directly at grain boundaries or induced by hydroxide ions adsorbed over the surface of the nanostructures. The results are in contradiction to the established role of defect states behind photoluminescent emission from ZnO nanostructures. In this work, mathematical modelling has been used to establish the dominant role that superoxide CT states can play in enhancing the efficiency of ZnO based optoelectronic devices such as light-emitting diodes and photodiodes.

SnO2/ZnO/p-Si and SnO2/TiO2/p-Si heterojunction UV photodiodes prepared using a hydrothermal method

Low dimensional nanostructures (NSs) exhibit superior properties for the fabrication of electronic and optoelectronic devices. The integration of these structures into the semiconductor devices is a novel efficient way to achieve high performance heterojunction UV photodiodes due to the enhanced light harvesting and facilitated charge-transfer. Herein, we report the effect of the type of metal oxide seed layers on the formation of SnO2NSs and the performance of SnO2/p-Si heterojunction UV photodiodes. The seed layers were fabricated by depositing ZnO and TiO2 thin films (TFs) on p-Si substrate by spin-coating method. The synthesis of SnO2NSs on each seed layers was achieved by hydrothermal growth technique. It was found that the morphology of nanostructures and light sensing performance of the heterojunction UV photodiodes were significantly influenced by type of seed layers. The photoresponsivity of our fabricated devices were found to be 68 mA/W for SnO2-NSs/ZnOTF/p-Si (device-1) and 92 mA/W for SnO2-NSs/TiO2-TF/p-Si (device-2). Thus, the device-2 is considered to be a better choice over the device-1 for UV detection applications. This research might shed light on new approaches to boost the performance of the heterojunction UV photodiodes.

(Invited) Theory of Silicon-Based Light Emitting

The indirect band gap preclude the silicon from making light-emitting devices, which is, however, a quest for exploration of large-scale integration of both electronic and photonic devices on a single microchip. Why silicon and germanium are both indirect band gap and most of group III-V and all group II-VI semiconductors are direct band gap is a long standing puzzle. In this talk, I will first present our recently developed unified theory, based on occupied d orbitals in cations, for explaining why diamond, Si, Ge, and Al-containing group III-V semiconductors have indirect band gap, and the remaining group III-V and II-VI semiconductors, except GaP, have direct band gap. This theory also unravels that it is impossible to make allotropes of Si and compounds consisting by Si mixed together with other elements high efficient direct bandgap light emitting. Interface and quantum confinement in low-dimensional Si nanostructures are two knobs to relax the momentum conservation law of optical transition. Our theory confirms that the light emission efficiency of Si quantum dots increases exponentially as reducing dot size, but its highest reachable efficiently is still two orders of magnitude smaller than that of direct bandgap InP and GaAs. Fortunately, we discovered that Si/Ge magic sequence superlattices exhibit orders more efficient at emission light than their existing counterpart records and approach more than 10% brightness of real direct gap materials, such as GaAs. Furthermore, regarding the direct bandgap is only 140 meV above the indirect bandgap in Germanium (Ge), it is well established that a tiny (1-2%) extensile strain could render an indirect-to-direct bandgap transition and makes it direct bandgap. However, there is no feasible way to create such extensive strain on Ge since the lattice parameter of Ge is even 4.3% larger than Si. We propose insert external atoms into interstitial sites of Ge, which grows on Si substrate through SiGe buffer layer, to induce an effective tensile uniaxial strain on Ge, a new pathway toward direct band gap Ge. Because our proposed scheme is highly compatible with CMOS fabrication process, our presented new pathway would be a solution for on-chip lasers.

Exciton diffusion in hBN -encapsulated monolayer MoSe2

Excitons, quasi particles composed of an electron and a hole, play an important role in optical responses in low-dimensional nanostructures. In this work, we have investigated exciton diffusion in a monolayer MoSe2 encapsulated between flakes of hexagonal boron nitrides (hBN/MoSe2/hBN). Through PL imaging and numerical solving the 2D diffusion equation, we revealed that temperature dependence of exciton mobility in the hBN/MoSe2/hBN shows non-saturating increase at low temperature, which is qualitatively different from those of quantum wells composed of compound semiconductors. Ultraflat structure of monolayer MoSe2 in the hBN/MoSe2/hBN probably leads to the suppression of charged-impurity scattering and surface-roughness scattering.

Low-Dimensional Nanostructures for Electrochemical Energy Applications

Materials with different nanostructures can have diverse physical properties, and they exhibit unusual properties as compared to their bulk counterparts. Therefore, the structural control of desired nanomaterials is intensely attractive to many scientific applications. In this brief review, we mainly focus on reviewing our recent reports based on the materials of graphene and the transition metal chalcogenide, which have various low-dimensional nanostructures, in relation to the use of electrocatalysts in electrochemical energy applications; moreover, related literatures were also partially selected for discussion. In addition, future aspects of the nanostructure design related to the further enhancement of the performance of pertinent electrochemical energy devices will also be mentioned.

Gate-Tunable Surface States in Topological Insulator β-Ag2Te with High Mobility.

Stimulated by novel properties in topological insulators, experimentally realizing quantum phases of matter and employing control over their properties have become a central goal in condensed matter physics. β-silver telluride (Ag2Te) is predicted to be a new type narrow-gap topological insulator. While enormous efforts have been plunged into the topological nature in silver chalcogenides, sophisticated research on low-dimensional nanostructures remains unexplored. Here, we report the record-high bulk carrier mobility of 298 600 cm2/(V s) in high-quality Ag2Te nanoplates and the coexistence of the surface and bulk state from systematic Shubnikov-de Haas oscillations measurements. By tuning the correlation between the top and bottom surfaces, we can effectively enhance the contribution of the surface to the total conductance up to 87% at 130 V. These results are instrumental to the high-mobility physics study and even suitable to explore exotic topological phenomena in this material system.

One-Dimensional Magnetic Order Stabilized in Edge-Reconstructed MoS2 Nanoribbon via Bias Voltage.

Low-dimensional nanostructures are expected to play an important role in spintronics. However, in low-dimensional systems, thermal fluctuations become more significant, which makes long-range magnetic ordering thermodynamically unfavorable. For example, as predicted by the Ising model, 1D magnetic order cannot survive, even at an arbitrary low finite temperature. In this study, utilizing a nanoproximity effect, we design a MoS2 nanoribbon material to stabilize 1D magnetic order without requiring the explicit application of an external magnetic field. The designed MoS2 nanoribbon has a new edge-reconstruction pattern, which is much more stable than previously reported structures. As a novel electronic property, one edge is nonmagnetic but conductive, and the opposite edge contains a magnetic moment in the predicted reconstruction pattern. Therefore, a bias voltage can drive a current along the former edge, which then generates a magnetic field at the opposite edge to stabilize the 1D magnetic order there. This result opens a new avenue to realize the integrated electrical control of magnetism.

Nanoconfined Transport Characteristic of Methane in Organic Shale Nanopores: The Applicability of the Continuous Model

In recent years, since the fast mass transport emerges from some low-dimensional nanostructures (e.g., graphene and CNTs), the applicability of the continuous model (e.g., hydrodynamics) for descri...

The effect of van der Waals force on the vibrational properties of low-dimensional nanostructure

On nanoscale, van der Waals (vdW) force has a very important influence on physical, chemical, electrical and mechanics properties of low-dimensional nanostructures. vdW force is an important factor affecting constitutive relation, boundary condition of low-dimensional nanostructures, and then affecting its natural frequency and nonlinear dynamics property. The nanostructures have wide applications in fabricating the nanoresonator and nanosensor, such as mass sensing, optical sensing, signal-processing, et al. In this paper, the latest research progress about the effect of vdW force on the vibrational properties of low-dimensional nanostructures is introduced. The main contents of this paper are as follows. The vdW force is introduced in the first part of this paper. The source of vdW force and the expression to describe vdW force are presented. The research progress on the basic development history of vdW force is discussed. In the second part, the effect of vdW force on the mechanical property of nanomaterials is discussed, including buckling, stretching, fracture, et al. vdW force is weak in resisting interlayer shear or sliding. However, it can affect the buckling and fracture behavior of nanomaterials, and can especially affect the deformation of nanomaterials. The effect of vdW force on the vibrational behavior of nanomaterials is presented in the third part. Different from the macro structure, vdW force has a significant influence on the vibrational behavior of low-dimensional nanoscale structures. The experimental method, molecular dynamic simulations method and continuum mechanics method are usually used for studying the influence of vdW force on the vibrational behavior of low-dimensional nanostructure. Due to the existence of vdW force between interlayer of low-dimensional nanostructures, the vibration of double-layered low-dimensional nanostructures appear anti-phase vibration mode and in-phase vibration mode. The frequency of anti-phase vibration mode is much higher than that of in-phase vibration mode, but the vdW force cannot affect the first order natural frequency. However, the existence of the vdW force between the interlayer of multi-layered low-dimensional nanostructure can cause the interlayer shear force. The vdW force can affect the vibrational behavior of low-dimensional nanostructure, e.g., the natural frequency is affected by vdW force. Some latest researches about the interlayer shear force caused by vdW force on the vibration property are discussed in this paper. The vdW force can also affect the boundary condition of the vibrational problems for low-dimensional nanostructure. For one-dimensional nanostructures, they are usually bridged on the substrate. But for two-dimensional nanostructures, they are usually suspended on the circular hole. The vdW force between the substrate and low-dimensional nanostructure is a very important factor that can affect the boundary conditions of vibrational property of these low-dimensional nanostructures. Some open problems and future researches in the dynamic behaviors of low-dimensional nanostructure with vdW force are presented. To study the effect of vdW force on the vibration characteristics of nanoscale structures by quantum mechanics method or semi quantum mechanics method is a prodigious challenge. The effect of nonlinear vdW force and long-range nonlocal vdW force on the vibration characteristics of nanoscale structures should be an interesting problem to be solved. The influence of vdW force on the vibration characteristics of nanostructures needs to be verified by experiments.

Substrate-Selective Morphology of Cesium Iodide Clusters on Graphene.

Formation and characterization of low-dimensional nanostructures is crucial for controlling the properties of two-dimensional (2D) materials such as graphene. Here, we study the structure of low-dimensional adsorbates of cesium iodide (CsI) on free-standing graphene using aberration-corrected transmission electron microscopy at atomic resolution. CsI is deposited onto graphene as charged clusters by electrospray ion-beam deposition. The interaction with the electron beam forms two-dimensional CsI crystals only on bilayer graphene, while CsI clusters consisting of 4, 6, 7, and 8 ions are exclusively observed on single-layer graphene. Chemical characterization by electron energy-loss spectroscopy imaging and precise structural measurements evidence the possible influence of charge transfer on the structure formation of the CsI clusters and layers, leading to different distances of the Cs and I to the graphene.

Ab Initio Study of Ferroelectric Critical Size of SnTe Low-Dimensional Nanostructures.

Beyond a ferroelectric critical thickness of several nanometers existed in conventional ferroelectric perovskite oxides, ferroelectricity in ultimately thin dimensions was recently discovered in SnTe monolayers. This discovery suggests the possibility that SnTe can sustain ferroelectricity during further low-dimensional miniaturization. Here, we investigate a ferroelectric critical size of low-dimensional SnTe nanostructures such as nanoribbons (1D) and nanoflakes (0D) using first-principle density-functional theory calculations. We demonstrate that the smallest (one-unit-cell width) SnTe nanoribbon can sustain ferroelectricity and there is no ferroelectric critical size in the SnTe nanoribbons. On the other hand, the SnTe nanoflakes form a vortex of polarization and lose their toroidal ferroelectricity below the surface area of 4 × 4 unit cells (about 25 Å on one side). We also reveal the atomic and electronic mechanism of the absence or presence of critical size in SnTe low-dimensional nanostructures. Our result provides an insight into intrinsic ferroelectric critical size for low-dimensional chalcogenide layered materials.

Effects of geometric variations on buckling properties of carbon nanostructures: A finite element analysis

Critical Bucking Load (CBL) is one of the essential parameters to describe the mechanic stabilities of materials, e.g. the low-dimensional carbon nanostructures. While the CBL of pristine carbon nanotubes have been previously investigated using the quantum mechanics-based calculations, the effect of geometrical variation on the CBL of carbon nanomaterials is rarely reported since it need considerably large atomic models, which needs high computational cost. In this study, both the analytical and Finite Element (FE) methods were employed to systematically explore the impact of atomic vacancies, shapes and heterostructures on the CBLs of carbon nanomaterials with the acceptable computational cost. Our studies on the pristine CNTs first demonstrate the validity of the method we used. After that, the systems with mono-/bi-/tri-/pinole-vacancies either on nanocones, nanotube, linearly-joined nanotubes or angle-adjoined were simulated and analyzed. Our results reveal that the CBL values decrease with the increase of the aspect ratio of all considered nanomaterials. Based on the obtained results, the CBL of nanocone with the aspect ratio of 1, reduces significantly, from 50 nN to 10 nN when the aspect ratio is 3. The CBL of homogeneous, capped, and joint CNTs reduces to below 2 nN when the aspect ratio is above 14. The introduction of geometric variations can greatly affect the CBL values. The larger atomic vacancy has more serious impact on the CBLs. The most highlighted impact is for the pinhole vacancy where the CBL reduces to up to 70% of the original value. Our studies on linearly-joint and angle-joint carbon hybrids further demonstrate that the CBL can also be affected by the boundary conditions and joint structures of the hybrids.

Interactions of Single Particle with Organic Matters: A Facile Bottom-Up Approach to Low Dimensional Nanostructures

A particle induces a pack of chemical reactions in nanospace: chemical reactions confined into extremely small space provide an ultimate technique for the nanofabrication of organic matter with a variety of functions. Since the discovery of particle accelerators, an extremely high energy density can be deposited, even by a single isolated particle with MeV-ordered kinetic energy. However, this was considered to cause severe damages to organic molecules due to its relatively small bond energies, and lack of ability to control the reactions precisely to form the structures while retaining physico-chemical molecular functionalities. Practically, the severely damaged area along a particle trajectory: a core of a particle track has been simply visualized for the detection/dosimetry of an incident particle to the matters, or been removed to lead nanopores and functionalized by refilling/grafting of fresh organic/inorganic materials. The use of intra-track reactions in the so-called “penumbra” or “halo” area of functional organic materials has been realized and provided us with novel and facile protocols to provide low dimensional nano-materials with perfect size controllability in the 21st century. These protocols are now referred to as single particle nanofabrication technique (SPNT) and/or single particle triggered linear polymerization technique (STLiP), paving the way towards a new approach for nanomaterials with desired functionalities from original molecules. Herein, we report on the extremely wide applicability of SPNT/STLiP protocols for the future development of materials for opto-electronic, catalytic, and biological applications among others.

Peptide-Driven Shape Control of Low-Dimensional DNA Nanostructures.

We report the rational design and fabrication of unusual low-dimensional DNA nanostructures through programmable and sequence-specific peptide interactions. Dual-bioactive block copolymers composed of DNA and amino acid-based polymers (DNA-b-poly(amino acid)) were synthesized by coupling oligonucleotides to phenylalanine (Phe)-based polymers. Unlike prototypical DNA block copolymers, which typically form simple spherical micelles, DNA-b-poly(amino acid) assemble into various low-dimensional structures such as nanofibers, ribbons, and sheets through controllable amino acid interactions. Moreover, DNA-b-poly(amino acid) assemblies can undergo protease-induced fiber-to-sheet shape transformations, where the morphology change is dictated by the type of enzymes and amino acid sequences. The peptide-based self-assembly reported here provides a programmable approach to fabricate dynamic DNA assemblies with diverse and unusual low-dimensional structures.

Nanocrystallization in FINEMET-Type Fe73.5Nb3Cu1Si13.5B9 and Fe72.5Nb1.5Mo2Cu1.1Si14.2B8.7 Thin Films.

A growing variety of microelectronic devices and magnetic field sensors as well as a trend of miniaturization demands the development of low-dimensional magnetic materials and nanostructures. Among them, soft magnetic thin films of Finemet alloys are appropriate materials for sensor and actuator devices. Therefore, one of the important directions of the research is the optimization of thin film magnetic properties. In this study, the structural transformations of the Fe73.5Nb3Cu1Si13.5B9 and Fe72.5Nb1.5Mo2Cu1.1Si14.2B8.7 films of 100, 150 and 200 nm thicknesses were comparatively analyzed together with their magnetic properties and magnetic anisotropy. The thin films were prepared using the ion-plasma sputtering technique. The crystallization process was studied by certified X-ray diffraction (XRD) methods. The kinetics of crystallization was observed due to the temperature X-ray diffraction (TDX) analysis. Magnetic properties of the films were studied by the magneto-optical Kerr microscopy. Based on the TDX data the delay of the onset crystallization of the films with its thickness decreasing was shown. Furthermore, the onset crystallization of the 150 and 200 nm films began at the temperature of about 400–420 °C showing rapid grain growth up to the size of 16–20 nm. The best magnetic properties of the films were formed after crystallization after the heat treatment at 350–400 °C when the stress relaxation took place.