Atomic-scale Devices Research Articles

A four-band perfect absorber based on high quality factor and high figure of merit of monolayer molybdenum disulfide

<sec> Transition-metal dichalcogenides with exceptional electrical and optical properties have emerged as a new platform for atomic-scale optoelectronic devices. However, the poor optical absorption resists their potential applications. In this paper, monolayer molybdenum disulfide four-band perfect absorber based on critical coupling and guided mode resonance is proposed theoretically and numerically by the finite difference time domain method. Meanwhile, the physical mechanism can be better analyzed through impedance matching and coupled mode theory. Monolayer molybdenum disulfide is placed between the silicon dioxide and a two-dimensional polymethyl methacrylate layer with a periodic square-shaped air groove structure. The three form a sandwich-like stacked structure similar to a rectangle. The bottom of the absorber uses a silver layer as the back reflection layer. Using the critical coupling principle of guided resonance, the high-efficiency light absorption of the monolayer molybdenum disulfide is obtained, that is, four perfect resonances are obtained at the resonance wavelengths (<i>λ</i><sub>1</sub> = 510.0 nm, <i>λ</i><sub>2</sub> = 518.8 nm, <i>λ</i><sub>3</sub> = 565.9 nm, and <i>λ</i><sub>4</sub> = 600.3 nm), the absorption rates are 99.03%, 98.10%, 97.30%, and 95.41%, and the average absorption rate is as high as 97.46% in the visible light spectrum range, which is over 12 times more than that of a bare monolayer MoS<sub>2</sub>. </sec><sec> The simulation results show that the adjusting of the geometric parameters of the structure can control the range of the resonance wavelength of the monolayer molybdenum disulfide, the system experiences three states, i.e. under-coupling, critical coupling, and over-coupling because of the leakage rate of resonance, thereby exhibiting advantageous tunability of operating wavelength in monolayer MoS<sub>2</sub>, which has important practical significance for improving the absorption intensity and selectivity of the monolayer molybdenum disulfide. The novel idea of using critical coupling to enhance the light-MoS<sub>2</sub> interaction can also be adopted in other atomically thin materials. At the same time, in this article the sensing performance of the absorber is discussed, and it is found that the highest quality factor, sensitivity and figure of merit of the sensor are 1294.1, 155.1 nm/RIU, and 436, respectively. The proposed structure is simple and the program is versatile. And these results indicate that the designed structure may offer a promising technology for improving the light-matter interaction in two-dimensional transition metal binary compounds, and has excellent application prospects in wavelength selective photoluminescence and photodetection. </sec>

Ask me anything: Michelle Simmons

Michelle Simmons is director of the Centre of Excellence for Quantum Computation and Communication Technology at the University of New South Wales, Australia. A pioneer in atomic-scale silicon devices, she has built the world’s smallest transistor, the narrowest conducting wires, 3D atomic electronics and the first two-qubit silicon gate. In 2018 Simmons was named Australian of the Year.

Electronic properties of hybrid WS2/MoS2 multilayer on flexible PET

Two-dimensional (2D) layered materials transition metal dichalcogenide compound (TMDC), which stack-together and form van der Waals heterostructures, have created interesting phenomena due to their interlayer interactions and their great potential for atomic-scale devices. Various electrical properties have been investigated. The presence of vacancies and their related charge trappings have been reported to affect the electrical properties. In this study, we investigate the electrical properties of hybrid WS2/MoS2 multilayer film deposited on polyethylene terephthalate (PET). The hybrid morphology and signatures are confirmed by the scanning electron microscope image and Raman shift spectra, respectively. We observed a semiconductor like behaviour as well as the large hysteresis which indicates the vacancies inducing charge trappings. This characteristics is different with the electronic characteristics of WS2 and MoS2 multilayer which tend to exhibit insulating behaviours and small hysteresis. This study shows how hybrid dichalcogenide WS2/MoS2 multilayer might create new features for future electronic devices.

Crystallographic orientation effect on cutting-based single atomic layer removal

The ever-increasing requirements for the scalable manufacturing of atomic-scale devices emphasize the significance of developing atomic-scale manufacturing technology. The mechanism of a single atomic layer removal in cutting is the key basic theoretical foundation for atomic-scale mechanical cutting. Material anisotropy is among the key decisive factors that could not be neglected in cutting at such a scale. In the present study, the crystallographic orientation effect on the cutting-based single atomic layer removal of monocrystalline copper is investigated by molecular dynamics simulation. When undeformed chip thickness is in the atomic scale, two kinds of single atomic layer removal mechanisms exist in cutting-based single atomic layer removal, namely, dislocation motion and extrusion, due to the differing atomic structures on different crystallographic planes. On close-packed crystallographic plane, the material removal is dominated by the shear stress-driven dislocation motion, whereas on non-close packed crystallographic planes, extrusion-dominated material removal dominates. To obtain an atomic, defect-free processed surface, the cutting needs to be conducted on the close-packed crystallographic planes of monocrystalline copper.

Control of energy dissipation in sliding low-dimensional materials

Frictional forces acting during the relative motion of nanosurfaces are the cause of energy loss and wear which limit an efficient assembly and yield of atomic-scale devices. In this research, we investigate the microscopic origin of the dissipative processes as a result of the frictional response, with the aim to control them in a subtle way. We recast the study of friction in terms of phonon modes of the system at the equilibrium, with no need to resort to dynamics simulations. As a case study, we here consider layer sliding in transition metal dichalcogenides thin films. We find that the population of specific atomic orbitals and the relative contribution of the atomic type to selected system vibrations are the crucial quantities which determine the frictional response in tribological conditions. A reduced amount of energy dissipation is found when the bond character is more ionic and the layer sliding is realized by a faster motion of the chalcogen atoms. The individuated relevant parameters governing the energy dissipation can be used as descriptors in high-throughput calculations or machine learning engines to screen databases of frictional materials. The presented framework is general and can be promptly extended to the design of tribological materials with targeted frictional response, irrespective of the chemistry and atomic topology.

Bipolar device fabrication using a scanning tunnelling microscope

Hydrogen-resist lithography with the tip of a scanning tunnelling microscope can be used to fabricate atomic-scale dopant devices in silicon substrates and could potentially be used to build a dopant-based quantum computer. However, all devices fabricated so far have been based on the n-type dopant precursor phosphine. Here, we show that diborane can be used as a p-type dopant precursor, allowing p-type and bipolar dopant devices to be created. Characterization of diborane δ-layers reveals that similar mobilities and densities can be achieved as for phosphine, with sheet resistivities as low as 300 Ω □−1. Scanning tunnelling microscope imaging and transport measurements of a 5.5-nm-wide p-type dopant nanowire give an estimated upper bound of 2 nm for the lithographic resolution of the p-type dopant profiles. By combining our p-type doping approach with established phosphine-based n-type doping, we fabricate a 100-nm-wide p–n junction and show that its electrical behaviour is similar to that of an Esaki diode. Hydrogen-resist lithography with the tip of a scanning tunnelling microscope can create p-type dopant nanowires and p–n junctions by using diborane as a p-type dopant precursor.

Assessing atomically thin delta-doping of silicon using mid-infrared ellipsometry

Abstract

Scanned Single-Electron Probe inside a Silicon Electronic Device.

Solid-state devices can be fabricated at the atomic scale, with applications ranging from classical logic to current standards and quantum technologies. Although it is very desirable to probe these devices and the quantum states they host at the atomic scale, typical methods rely on long-ranged capacitive interactions, making this difficult. Here, we probe a silicon electronic device at the atomic scale using a localized electronic quantum dot induced directly within the device at a desired location, using the biased tip of a low-temperature scanning tunneling microscope. We demonstrate control over short-ranged tunnel coupling interactions of the quantum dot with the device's source reservoir using sub-nanometer position control of the tip and the quantum dot energy level using a voltage applied to the device's gate reservoir. Despite the ∼1 nm proximity of the quantum dot to the metallic tip, we find that the gate provides sufficient capacitance to enable a high degree of electric control. Combined with atomic-scale imaging, we use the quantum dot to probe applied electric fields and charge in individual defects in the device. This capability is expected to aid in the understanding of atomic-scale devices and the quantum states realized in them.

The sub-band structure of atomically sharp dopant profiles in silicon

The downscaling of silicon-based structures and proto-devices has now reached the single-atom scale, representing an important milestone for the development of a silicon-based quantum computer. One especially notable platform for atomic-scale device fabrication is the so-called Si:P δ-layer, consisting of an ultra-dense and sharp layer of dopants within a semiconductor host. Whilst several alternatives exist, it is on the Si:P platform that many quantum proto-devices have been successfully demonstrated. Motivated by this, both calculations and experiments have been dedicated to understanding the electronic structure of the Si:P δ-layer platform. In this work, we use high-resolution angle-resolved photoemission spectroscopy to reveal the structure of the electronic states which exist because of the high dopant density of the Si:P δ-layer. In contrast to published theoretical work, we resolve three distinct bands, the most occupied of which shows a large anisotropy and significant deviation from simple parabolic behaviour. We investigate the possible origins of this fine structure, and conclude that it is primarily a consequence of the dielectric constant being large (ca. double that of bulk Si). Incorporating this factor into tight-binding calculations leads to a major revision of band structure; specifically, the existence of a third band, the separation of the bands, and the departure from purely parabolic behaviour. This new understanding of the band structure has important implications for quantum proto-devices which are built on the Si:P δ-layer platform.

Controlled removal of hydrogen atoms from H-terminated silicon surfaces

The controlled formation of dangling bond structures on a H-terminated silicon surface is the first step in an atomically precise method of fabrication of silicon quantum electronic devices. An ultrahigh vacuum scanning tunneling microscope (STM) tip is used to selectively desorb hydrogen atoms from a Si(100)-2×1:H surface by injecting electrons with the sample held at a positive bias voltage. The authors propose a lithography method that allows the STM to operate under negative bias imaging conditions and simultaneously desorb H atoms as required. A high frequency signal is added to the negative bias voltage to deliver the required energy for hydrogen removal. The resulting current at this frequency and its harmonics are filtered to minimize their effect on the operation of the STM’s feedback control loop. The authors show that the chance of tip-sample crash during the lithography process is reduced by employing this method. They also demonstrate that this approach offers a significant potential for controlled and precise removal of H atoms from a H-terminated silicon surface and thus may be used for the fabrication of practical silicon-based atomic-scale devices.

Rake angle effect in cutting-based single atomic layer removal

When minimum cutting depth is down to a single atomic layer, two portions of the cutting tool, namely, cutting edge and lowest atoms of the cutting tool, are involved in the cutting-induced material removal. Correspondingly, there are different critical rake angles for those two portions of the tool, different from the nominal rake angle in conventional cutting and edge radius-induced effective rake angle in nanocutting. Both they should be considered in atomic and close-to-atomic cutting to obtain the defect-free processed surface with an ideal crystalline structure. Molecular dynamics modelling is carried out to investigate the critical rake angles to enable single atomic layer removal on monocrystalline Cu (1 1 1) surface. The analysis results clearly indicate that the critical rake angles of nanometric cutting edge and the lowest tool atoms for single atomic layer removal are among the range of (–70°, –65°) and (–17°, –14°), respectively. To achieve single atomic layer removal, the tool edge radius is suggested to be not greater than 2 nm. The research findings would provide theoretical guidelines to the cutting tool design for the application of mechanical cutting of high-performance atomic scale devices.

Periodic energy decomposition analysis for electronic transport studies as a tool for atomic scale device manufacturing

Atomic scale manufacturing is a necessity of the future to develop atomic scale devices with high precision. A different perspective of the quantum realm, which includes the tunnelling effect, leakage current at the atomic-scale, Coulomb blockade and Kondo effect, is inevitable for the fabrication and hence, the mass production of these devices. For these atomic-scale device development, molecular level devices must be fabricated. Proper theoretical studies could be an aid towards the experimental realities. Electronic transport studies are the basis to realise and interpret the problems happening at this minute scale. Keeping these in mind, we present a periodic energy decomposition analysis (pEDA) of two potential candidates for moletronics: phthalocyanines and porphyrins, by placing them over gold substrate cleaved at the (111) plane to study the adsorption and interaction at the interface and then, to study their application as a channel between two electrodes, thereby, providing a link between pEDA and electronic transport studies. pEDA provides information regarding the bond strength and the contribution of electrostatic energy, Pauli’s energy, orbital energy and the orbital interactions. Combining this analysis with electronic transport studies can provide novel directions for atomic/close-to-atomic-scale manufacturing (ACSM). Literature survey shows that this is the first work which establishes a link between pEDA and electronic transport studies and a detailed pEDA study on the above stated molecules. The results show that among the molecules studied, porphyrins are more adsorbable over gold substrate and conducting across a molecular junction than phthalocyanines, even though both molecules show a similarity in adsorption and conduction when a terminal thiol linker is attached. A further observation establishes the importance of attractive terms, which includes interaction, orbital and electrostatic energies, in correlating the pEDA study with the transport properties. By progressing this research, further developments could be possible in atomic-scale manufacturing in the future.

Hydrogen inserted into the Si(100)-2 × 1-H surface: a first-principles study.

Hydrogen can be inserted into Si(100)-2 × 1-H during surface preparation or during the hydrogen desorption lithography used to create atomic-scale devices. Here, a hydrogen atom inserted into a hydrogen monolayer on the Si(100)-2 × 1 surface has been studied using density functional theory. Hydrogen-induced defects were considered in their neutral, negative, and positive charge states. It was found that hydrogen forms a dihydride unit on the surface in the most stable neutral and negative charge states. Hydrogen located in the groove between dimer rows is also one of the most stable negative charge states. In the positive charge state, hydrogen forms a three-center bond inside a Si dimer, Si-H-Si, similar to the bulk case. A comparison of simulated scanning tunneling microscopy (STM) images with the experimental data available in the literature showed that neutral and negatively charged hydrogen-induced defects were already observed in experiments. The results reveal that the H atom inserted into a hydrogen monolayer on the Si(100)-2 × 1 surface can lead to the formation of a positively or negatively charged defect. It is shown that H atoms in the considered configurations can play a role in various surface reactions.

Heteroepitaxial vertical perovskite hot-electron transistors down to the monolayer limit

Two-dimensional heterostructures combined with vertical geometries are candidates to probe and utilize the physical properties of atomically-thin materials. The vertical configuration enables a unique form of hot-carrier spectroscopy as well as atomic-scale devices. Here, we present the room-temperature evolution of heteroepitaxial perovskite hot-electron transistors using a SrRuO3 base down to the monolayer limit (∼4 Å). As a fundamental electronic probe, we observe an abrupt transition in the hot-electron mean free path as a function of base thickness, coinciding with the thickness-dependent resistive transition. As a path towards devices, we demonstrate the integrated synthesis of perovskite one-dimensional electrical edge contacts using water-soluble and growth-compatible Sr3Al2O6 hard masks. Edge-contacted monolayer-base transistors exhibit on/off ratios reaching ∼108, complete electrostatic screening by the base manifesting pure hot-electron injection, and excellent scaling of the output current density with device dimensions. These results open new avenues for incorporating emergent phenomena at oxide interfaces and in heterostructures.

(Invited) Energy Transport at the Nanoscale

Energy transport at the nanoscale differs fundamentally from that at the bulk scale. Characterization and understanding of the unique energy transport phenomena that arise at the nanoscale is key to developing novel technological applications such as energy conversion devices and atomic-scale device components. In this talk, I will present my work on understanding unique thermal transport in various nanomaterials. I will also discuss heat dissipation in atomic-scale junctions that are composed of single atomic chains or single molecules. Further, experimental techniques to perform challenging experiments will be briefly described.

A STEM-based Path Towards Atomic-scale Silicon-based Devices

A STEM-based Path Towards Atomic-scale Silicon-based Devices

Room-temperature ferroelectricity in MoTe2 down to the atomic monolayer limit

Ferroelectrics allow for a wide range of intriguing applications. However, maintaining ferroelectricity has been hampered by intrinsic depolarization effects. Here, by combining first-principles calculations and experimental studies, we report on the discovery of robust room-temperature out-of-plane ferroelectricity which is realized in the thinnest monolayer MoTe2 with unexploited distorted 1T (d1T) phase. The origin of the ferroelectricity in d1T-MoTe2 results from the spontaneous symmetry breaking due to the relative atomic displacements of Mo atoms and Te atoms. Furthermore, a large ON/OFF resistance ratio is achieved in ferroelectric devices composed of MoTe2-based van der Waals heterostructure. Our work demonstrates that ferroelectricity can exist in two-dimensional layered material down to the atomic monolayer limit, which can result in new functionalities and achieve unexpected applications in atomic-scale electronic devices.

Atomically thin planar metasurfaces

Enhancing light absorption in two-dimensional (2-D) materials using simple planar structures is important for the development of large-area atomic-scale photonic devices but also has been a major challenge. We theoretically present a general strategy to create an atomically thin planar metasurface consisting of a semiconductor monolayer (replacing the array of subwavelength elements in classical metasurfaces), a transparent spacer, and a metallic back reflector, to achieve near-perfect, polarization-sensitive absorption in 2-D materials. This strategy is validated by experimentally demonstrating planar metasurfaces based on atomically thin Ge films. Higher than 90% exclusive absorption, 2.5 times greater than prior demonstration, is achieved in the MoS2 monolayer for TE polarization. Our strategy can be extended to the infrared spectrum and thus has potential for thermal energy applications.

Si and Ge based metallic core/shell nanowires for nano-electronic device applications

One dimensional heterostructure nanowires (NWs) have attracted a large attention due to the possibility of easily tuning their energy gap, a useful property for application to next generation electronic devices. In this work, we propose new core/shell NW systems where Ge and Si shells are built around very thin As and Sb cores. The modification in the electronic properties arises due to the induced compressive strain experienced by the metal core region which is attributed to the lattice-mismatch with the shell region. As/Ge and As/Si nanowires undergo a semiconducting-to-metal transition on increasing the diameter of the shell. The current-voltage (I-V) characteristics of the nanowires show a negative differential conductance (NDC) effect for small diameters that could lead to their application in atomic scale device(s) for fast switching. In addition, an ohmic behavior and upto 300% increment of the current value is achieved on just doubling the shell region. The resistivity of nanowires decreases with the increase in diameter. These characteristics make these NWs suitable candidates for application as electron connectors in nanoelectronic devices.

Size effect for achieving high mechanical performance body-centered cubic metals and alloys

Submicron and nanostructured body-centered cubic (BCC) metals exhibit unusual mechanical performance compared to their bulk coarse-grained counterparts, including high yield strength and outstanding ductility. These properties are important for their applications in micro-, nano- and even atomic-scale devices as well as for their usages as components for enhancing the performances of structural materials. One aspect of the unusual mechanical properties of small-sized BCC metals is closely related to their dimensional confinement. Decreasing the dimensions of single crystalline metals or the grain sizes of polycrystalline metals contributes significantly to the strengthening of the small-sized BCC metals. In the last decade, significant progress has been achieved in understanding the plasticity and deformation behaviors of small-sized BCC metals. This paper aims to provide a comprehensive review on the current understanding of size effects on the plasticity and deformation mechanisms of small-sized BCC metals. The techniques used for in situ characterization of the deformation behavior and mechanical properties of small-sized samples are also presented.