Low-dimensional Nanomaterials Research Articles

Research progresses of resistive random access memory

Resistive random access memory (RRAM) is one of the most promising candidates of next-generation non-volatile memories. Comparing with the traditional floating gate Flash memory, RRAM has advantages in cell structure, operation speed, scalability and ease of 3D integration. In this paper, the RRAM technology is systematically reviewed. The operation principle, technological advantages, development history and fundamental scientific issues of RRAM are introduced. Then several key issues in RRAM are summarized and analyzed in detail, including the materials system, resistive switching mechanism, novel physical effects in RRAM, and 2D/3D integration of RRAM. For the materials system in RRAM, traditional resistive switching films, including solid-electrolyte, complex transition metal oxide, binary metal oxide, are first summarized, following with the analyzing of the recent research progress of the applications of nanowires and 2D materials in RRAM devices. These low-dimensional nanomaterials can be used as the switching dielectrics, planar electrode/nano-electrode/edge electrode and can play active role in the improvement of scalability, switching performances and switching uniformity. Next, we summarize and analyze three resistive switching mechanisms in filamentary RRAM device, i.e. thermochemical mechanism (TCM), electrochemical metallization (ECM) mechanism, valence change mechanism (VCM). Their corresponding SET/RESET switching principles and fundamental electrical characteristics, experimental verifications by transmission electron microscope (TEM) observation are elucidated in detail. Subsequently, based on the resistive switching mechanisms, the novel physical phenomena in RRAM induced by the change of the material microstructure and energy band in the process of resistive switching are elaborated. Three physical effects, including conductance quantization effect, magnetoelectric effect, photoelectric effect, are successively analyzed. At last, considering the integration technology of RRAM, we summarize and analyze the key scientific issues in memory array architecture, 2D/3D integration.

(Invited) Synthesis and Applications of Novel Two-Dimensional Nanomaterials

In this talk, I will summarize the recent research on synthesis, characterization and applications of two-dimensional nanomaterials in my group. I will introduce the synthesis and characterization of novel low-dimensional nanomaterials, such as graphene-based composites, and the epitaxial growth of metal and semiconductor nanostructures on solution-processable transition metal dichalcogenide (TMD) nanoshees at ambient conditions, single- or few-layer metal dichalcogenide nanosheets and hybrid nanomaterials, other 2D nanomaterials. Then I will demonstrate the applications of these novel nanomaterials in electronic devices.

Building Pentagons into Graphenic Structures by On-Surface Polymerization and Aromatic Cyclodehydrogenation of Phenyl-Substituted Polycyclic Aromatic Hydrocarbons

Five-membered carbon rings play a crucial role in determining the geometric structures and electronic properties of carbon-based materials. However, incorporating five-membered rings into low-dimensional carbon nanomaterials with atomic precision is still a challenge. Here, we report on the on-surface synthesis of a one-dimensional conjugated polymer comprising fluoranthene subunits obtained by the surface-assisted aromatic cyclodehydrogenation of 1-phenylnaphthalene moieties within the precursor monomers. By using scanning tunneling microscopy (STM), we investigate the formation of the polymer by thermally induced dehalogenative aryl–aryl coupling and subsequent aromatic cyclodehydrogenation on a Au(111) substrate. The formed five-membered carbon rings are directly observed with atomic resolution by noncontact atomic force microscopy (nc-AFM). Our results show that carefully designed phenyl-substituted polycyclic aromatic hydrocarbons can be used for the rational fabrication of five-membered rings in the...

Piezotronics and piezo-phototronics for adaptive electronics and optoelectronics

Low-dimensional piezoelectric semiconductor nanomaterials, such as ZnO and GaN, have superior mechanical properties and can be integrated into flexible devices that can be subjected to large strain. More importantly, the coupling between piezoelectric polarization and semiconductor properties (for example, electronic transport and photoexcitation) in these materials gives rise to unprecedented device characteristics. This has increased research interest in the emerging fields of piezotronics and piezo-phototronics, which offer new means of manipulating charge-carrier transport, generation, recombination or separation in the controlled operation of flexible devices through the application of external mechanical stimuli. We review the recent progress in advancing our fundamental understanding and in realizing practical applications of piezotronics and piezo-phototronics, and provide an in-depth discussion of future research directions. Piezotronics and piezo-phototronics offer new means of implementing adaptive electronics and optoelectronics, taking advantage of the coupling between piezoelectric polarization and semiconductor properties in piezoelectric semiconductor nanomaterials. This Review discusses the recent progress in piezotronics and piezo-phototronics, as well as future research directions.

Progress on synthesis, functionalisation and applications of graphene nanoplatelets

Carbon is the fifteenth most abundant element in Earth's crust, and the fourth most abundant element in the universe. Carbon nanostructures, or nanocarbons, i.e. the low-dimensional nanomaterials, are being extensively researched for the past two decades because of their unique structure and electronic properties, prompting a huge interest in its fundamental research and applications in molecular electronics, materials science, energy storage and conversion, bio-medicine, sensing, and bio-sensing. Graphene was recently touted as a wonder material, because of its high-mechanical strength, high-electron mobility, lightness, flexibility, single-atom thickness, and near-transparency. These properties make graphene a very promising material for composites, thin films, electromagnetic shielding, barrier films, and sensors. This review focuses on the latest progress on the preparation and functionalisation of graphene nanoplatelets, and discusses its potential applications, future prospects, and challenges in the context of theranostic applications.

Charge Transfer Dynamics from Photoexcited Semiconductor Quantum Dots.

Understanding photoinduced charge transfer from nanomaterials is essential to the many applications of these materials. This review summarizes recent progress in understanding charge transfer from quantum dots (QDs), an ideal model system for investigating fundamental charge transfer properties of low-dimensional quantum-confined nanomaterials. We first discuss charge transfer from QDs to weakly coupled acceptors within the framework of Marcus nonadiabatic electron transfer (ET) theory, focusing on the dependence of ET rates on reorganization energy, electronic coupling, and driving force. Because of the strong electron-hole interaction, we show that ET from QDs should be described by the Auger-assisted ET model, which is significantly different from ET between molecules or from bulk semiconductor electrodes. For strongly quantum-confined QDs on semiconductor surfaces, the coupling can fall within the strong coupling limit, in which case the donor-acceptor interaction and ET properties can be described by the Newns-Anderson model of chemisorption. We also briefly discuss recent progress in controlling charge transfer properties in quantum-confined nanoheterostructures through wavefunction engineering and multiple exciton dissociation. Finally, we identify a few key areas for further research.

Hybrid Modeling of Molecular Sensing and Catalysis in Low-dimensional Nanomaterials

We use hybrid quantum and classical modeling to describe recently observed molecular sensing at graphene grain boundaries and electrochemical reduction of carbon dioxide (CO2) on molybdenum disulphide (MoS2) flakes. In the sensing studies, classical and quantum molecular dynamics simulations are used to relax graphene with grain boundaries deposited on amorphous SiO2. Electronic structure calculations show how this graphene is locally doped by the substrate and adsorbed molecules, while electronic transport modelling reveals that the doping can lead to synchronous opening and closing of local electron transport channels, resulting in a very large observed sensitivity. In the catalysis studies, electronic structure calculations combined with ab initio molecular dynamics uncover that the metallic character and high d-electron density of molybdenum-terminated MoS2 edges and EMIM-ion delivery are responsible for the observed superior CO2 reduction performance, with a high current density and low ∼54mV overpotential. The described mechanisms open up new pathways for the design of nanometer-scale highly sensitive chemical detectors and the development of inexpensive systems of CO2 conversion to energy-rich products. These studies illustrate how hybrid modeling techniques can explain complex transport phenomena in nanostructures.

Gap controlled plasmon-dielectric coupling effects investigated with single nanoparticle-terminated atomic force microscope probes.

Precise positioning of a plasmonic nanoparticle (NP) near a small dielectric surface is not only necessary for understanding gap-dependent interactions between a metal and dielectric but it is also a critical component in building ultrasensitive molecular rulers and force sensing devices. In this study we investigate the gap-dependent scattering of gold and silver NPs by controllably depositing them on an atomic force microscope (AFM) tip and monitoring their scattering within the evanescent field of a tin dioxide nanofiber waveguide. The enhanced distance-dependent scattering profiles due to plasmon-dielectric coupling effects show similar decays for both gold and silver NPs given the strong dependence of the coupling on the decaying power in the near-field. Experiments and simulations also demonstrate that the NPs attached to the AFM tips act as free NPs, eliminating optical interference typically observed from secondary dielectric substrates. With the ability to reproducibly place individual plasmonic NPs on an AFM tip, and optically monitor near-field plasmon-dielectric coupling effects, this approach allows a wide-variety of light-matter interactions studies to be carried out on other low-dimensional nanomaterials.

Edge States and Topological Insulating Phases Generated by Curving a Nanowire with Rashba Spin-Orbit Coupling

We prove that curvature effects in low-dimensional nanomaterials can promote the generation of topological states of matter by considering the paradigmatic example of quantum wires with Rashba spin-orbit coupling, which are bent in a nanoscale periodic serpentine structure. The effect of the periodic curvature generally results in the appearance of insulating phases with a corresponding novel butterfly spectrum characterized by the formation of finite measure complex regions of forbidden energies. When the Fermi energy lies in the gaps, the system displays localized end states protected by topology. We further show that for certain superstructure periods the system possesses topologically nontrivial insulating phases at half filling. Our results suggest that the local curvature and the topology of the electronic states are inextricably intertwined in geometrically deformed nanomaterials.

Analysing black phosphorus transistors using an analytic Schottky barrier MOSFET model

Owing to the difficulties associated with substitutional doping of low-dimensional nanomaterials, most field-effect transistors built from carbon nanotubes, two-dimensional crystals and other low-dimensional channels are Schottky barrier MOSFETs (metal-oxide-semiconductor field-effect transistors). The transmission through a Schottky barrier-MOSFET is dominated by the gate-dependent transmission through the Schottky barriers at the metal-to-channel interfaces. This makes the use of conventional transistor models highly inappropriate and has lead researchers in the past frequently to extract incorrect intrinsic properties, for example, mobility, for many novel nanomaterials. Here we propose a simple modelling approach to quantitatively describe the transfer characteristics of Schottky barrier-MOSFETs from ultra-thin body materials accurately in the device off-state. In particular, after validating the model through the analysis of a set of ultra-thin silicon field-effect transistor data, we have successfully applied our approach to extract Schottky barrier heights for electrons and holes in black phosphorus devices for a large range of body thicknesses.

Tuning strain in flexible graphene nanoelectromechanical resonators

The structural flexibility of low dimensional nanomaterials offers unique opportunities for studying the impact of strain on their physical properties and for developing innovative devices utilizing strain engineering. A key towards such goals is a device platform which allows the independent tuning and reliable calibration of the strain. Here, we report the fabrication and characterization of graphene nanoelectromechanical resonators (GNEMRs) on flexible substrates. Combining substrate bending and electrostatic gating, we achieve the independent tuning of the strain and sagging in graphene and explore the nonlinear dynamics over a wide parameter space. Analytical and numerical studies of a continuum mechanics model, including the competing higher order nonlinear terms, reveal a comprehensive nonlinear dynamics phase diagram, which quantitatively explains the complex behaviors of GNEMRs.

Reliable and cost effective design of intermetallic Ni2Si nanowires and direct characterization of its mechanical properties.

We report that a single crystal Ni2Si nanowire (NW) of intermetallic compound can be reliably designed using simple three-step processes: casting a ternary Cu-Ni-Si alloy, nucleate and growth of Ni2Si NWs as embedded in the alloy matrix via designing discontinuous precipitation (DP) of Ni2Si nanoparticles and thermal aging, and finally chemical etching to decouple the Ni2Si NWs from the alloy matrix. By direct application of uniaxial tensile tests to the Ni2Si NW we characterize its mechanical properties, which were rarely reported in previous literatures. Using integrated studies of first principles density functional theory (DFT) calculations, high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDX) we accurately validate the experimental measurements. Our results indicate that our simple three-step method enables to design brittle Ni2Si NW with high tensile strength of 3.0 GPa and elastic modulus of 60.6 GPa. We propose that the systematic methodology pursued in this paper significantly contributes to opening innovative processes to design various kinds of low dimensional nanomaterials leading to advancement of frontiers in nanotechnology and related industry sectors.

Ferritin-mixed solution plasma system yielding low-dimensional carbon nanomaterials and their application to flexible conductive paper

We demonstrate the production of low-dimensional carbon nanomaterials using a solution plasma system and their application to flexible conductive paper. The solution plasma system consists of two graphite electrodes and a beaker filled with ferritin-mixed deionized water. Ferritin molecules are used as the growth catalyst of the carbon nanomaterials. A high voltage of 15 kV at a frequency of 25 kHz is supplied to the electrodes using an alternating-current power source. The effects of the graphite rod diameters and the concentration of ferritin molecules are comparatively investigated. The produced carbon nanomaterials are characterized using Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. We confirmed the synthesis of graphitic platelets, onion-like structures, and carbon nanotubes. Finally, we fabricated flexible conductive papers using the produced materials with a good electrical conductance.

Computing Equilibrium Shapes of Wurtzite Crystals: The Example of GaN.

Crystal morphologies are important for the design and functionality of devices based on low-dimensional nanomaterials. The equilibrium crystal shape (ECS) is a key quantity in this context. It is determined by surface energies, which are hard to access experimentally but can generally be well predicted by first-principles methods. Unfortunately, this is not necessarily so for polar and semipolar surfaces of wurtzite crystals. By extending the concept of Wulff construction, we demonstrate that ECSs can nevertheless be obtained for this class of materials. For the example of GaN, we identify different crystal shapes depending on the chemical potential, shedding light on experimentally observed GaN nanostructures.

On-surface synthesis of carbon-based scaffolds and nanomaterials using terminal alkynes.

The covalent linking of acetylene compounds is an important synthetic tool to control carbon-carbon bond formation and has been extensively studied for more than a century. Notably, Glaser coupling and subsequently developed refined procedures present an important route for the fabrication of distinct carbon-based scaffolds incorporating units with both sp(2)- and sp-hybridizations, such as carbyne chains, or two-dimensional (2D) graphyne or graphdiyne networks. However, the realization of the envisioned regular low-dimensional compounds and nanoarchitectures poses formidable challenges when following conventional synthesis protocols in solution, which we briefly overview. Now, recent developments in on-surface synthesis establish novel means for the construction of tailored covalent nanostructures under ultrahigh vacuum conditions. Here we focus on the exploration of pathways utilizing interfacial synthesis with terminal alkynes toward the atomically precise fabrication of low-dimensional carbon-rich scaffolds and nanomaterials. We review direct, molecular-level investigations, mainly relying on scanning probe microscopy, providing atomistic insights into thermally activated reaction schemes, their special pathways and products. Using custom-made molecular units, the employed homocoupling, cyclotrimerization, cycloaddition, and radical cyclization processes indeed yield distinct compounds, extended oligomers or 2D networks. Detailed insights into surface interactions such as bonding sites or conformational adaptation, and specific reaction mechanisms, including hierarchic pathways, were gained by sophisticated density functional theory calculations, complemented by X-ray spectroscopy measurements. For the fabrication of regular nanostructures and architectures, it is moreover imperative to cope with spurious side reactions, frequently resulting in chemical diversity. Accordingly, we highlight measures for increasing chemo- and regioselectivity by smart precursor design, substrate templating, and external stimuli. The ensuing preorganization of functional groups and control of side reactions increases product yields markedly. Finally, the electronic band structures of selected cases of novel low-dimensional hydrocarbon materials accessible with the monomers employed to date are discussed with a specific focus on their differences to theoretically established graphyne- and graphdiyne-related scaffolds. The presented methodology and gained insights herald further advancements in the field, heading toward novel molecular compounds, low-dimensional nanostructures, and coherently reticulated polymeric layers, eventually presenting well-defined arrangements with specific carbon-carbon bond sequencing and electronic characteristics. The functional properties of these or other foreseeable scaffolds and architectures bear significant prospects for a wide range of applications, for example, in nanoelectronics, photonics, or carbon-based technologies.

(Invited) Wearable Electronics Using Low-Dimensional Nanomaterials

Transparent conductive electrodes (TCEs), which have transparency and conductivity, are of increasing significance and demand for diverse application areas such as information (displays, touch screen), energy (solar cell, architectural), and environment (sensors). Nowadays, indium tin oxide (ITO) deposited by sputtering process is used as the main TCE material. Although the ITO presents low sheet resistance (~ 30 Ohm/sq) and high transparency (~ 90 %), its fragility limits many potential applications in flexible and stretchable electronics. For these reason, many types of alternative materials such as conducting polymers, carbon nanotubes, graphene, and metal nanowires, have been studied as candidates of the flexible and stretchable TCEs. Although many of these candidates exhibit good mechanical flexibility and stretchability, none shows significantly higher conductivity and transparency together, compared to ITO. Here, we present a simple fabrication process of high-performance, stretchable TCEs based on the metal nanofibers which have long and continuous web geometries, hybrid with 2D material graphene. These TCEs show superb electric conductivity (~ 1 Ohm/sq) with high transparency (~ 91 %) as well as substantial flexibility and stretchability. We believe these TCEs based on the nanostructures present a promising strategy toward flexible and wearable electronics, and indicate the promise of future electronics beyond the limits of conventional ITO.

(Invited) Enhanced Energy Harvesting Scheme Utilizing Hierarchical Micro/Nanostructures

In this talk, I will introduce the development of low dimensional nanomaterials in my group and its applications on solar cell and solar energy storage. In the first part of my talk, I will introduce the development of hierarchical nanostructures used to enhance the energy harvesting based on photo-solar and thermal-solar in my group. First, I will report fabrication of a large area Cu(In,Ga)Se2 nanotip arrays (CIGS NTRs) by using one step Ar+ milling process without template. By controlling milling time and incident angles, the length of CIGS NTRs with adjustable tilting orientations can be precisely controlled. The CIGS NTRs have very low reflectance < 0.1 % at incident wavelengths between 300 nm to 1200 nm. Open circuit voltage and short circuit current of CIGS NTRs solar cell were measured to be ~390 mV and ~22.56 mA/cm2, yielding the filling factor and the efficiency of 59 % and 5.2 %, respectively. In contrast to CIGS thin film solar cell with efficiency of 3.2 %, the nanostructured CIGS NTRs can have efficiency enhancement of ~160 % due to the higher light absorption ability because of the nanostructure. In the 2nd part of my talk, an enhanced heat capacity of modified HITEC molten salt by doping of metal NPs will be reported. We encapsulate the Tin alloy into Sn/SiOx core-shell NPs to prevent mixture with salt when operation temperature is higher than the melting point of Tin. The SiOx shell layer can protect the Sn NPs to be oxidized and maintains a melted status during the heat cycles, providing a stable leant heat value of 28.36 J/g. Besides, effective heat capacity of HITEC salt with Sn/SiOx core-shell NPs can be enhanced to be 30 % a mixture percentage of 5 wt%. The merits of this approach show a low-cost and easy way to enhance the thermal properties of current molten salt without complicated manufacturing.

Silver catalyzed gallium phosphide nanowires integrated on silicon and in situ Ag-alloying induced bandgap transition

In this work, we demonstrate a silver catalyzed heteroepitaxial growth of gallium phosphide nanowires (GaP NWs) on silicon. The morphology and growth direction of GaP NWs on differently orientated Si substrates were investigated. From crystallographic analysis, we inferred that Ag from catalyst is incorporated into the GaP during the chemical beam epitaxy (CBE) process. Using the PL spectrum and time-resolved emission spectroscopy, the optical properties of Ag-catalyzed GaP NWs were greatly modified, with bandgap transitions in the blue range. The Raman characterizations further confirmed the Ag incorporation into GaP during the growth. From the bandgap calculations, it was deduced that Ag was substituted on the Ga site with bandgap broadening. The in situ Ag-alloying during the growth of Ag-catalyzed GaP NWs greatly modified the band structure of GaP, and could lead to further applications in optoelectronics for low-dimensional GaP-based nanomaterials.

Exploring the Self-Assembly Behaviors of an Organic Molecule Functionalized by Terminal Alkyne and Aldehyde Groups on Au(111)

On-surface self-assembly from molecular building blocks directed by supramolecular interactions has been widely reckoned as an efficient method for controllable construction of low-dimensional nanostructures and nanomaterials. Numerous efforts have been devoted to exploring the self-assembled behaviors of molecular precursors on different surfaces and unravelling the underlying mechanism. Generally, the molecular precursors are functionalized with one kind of functional groups for directing the self-assembly. In this study, by combining real-space direct visualization and DFT calculations, we have investigated the self-assembly behaviors of an organic molecule functionalized by two different functional groups: terminal alkyne and aldehyde groups on Au(111). An ordered racemic island nanostructure is formed on Au(111), which results from the hybrid interactions between the two functional groups. Detailed DFT calculations have been performed to compare the different binding ways and binding strengths betwee...

(Invited) Single-Walled Carbon Nanotube Spectroscopy in Complex Environments

The real-time and spatially-resolved detection and identification of analytes present important goals for next-generation nanoscale sensors. Low-dimensional nanomaterials are improving the detection of molecular binding phenomena. The resulting optical technologies may allow for the quantification of disease biomarkers and other bioanalytes. The photoluminescence of single-walled carbon nanotubes may facilitate quantification of analytes within complex environments. To build optical probes using these materials, we are developing new spectroscopic techniques to detect nanotube emission in biological samples and new polymers to control their optical properties.