Sheet Of Atoms Research Articles

Physical vapor deposited 2D bismuth for CMOS technology

Two-dimensional (2D) bismuth, bismuthene, is an emerging pnictogen family member that has received increasing research attention in the past few years, which could yield exotic electrical, thermal, and optical properties due to unique band structure. This review provides a holistic view of recent research advances on 2D bismuth material synthesis and device applications in complementary metal oxide semiconductor (CMOS) technology. Firstly, the atomic and band structure of bismuthene is reviewed as the fundamental understanding of its physical properties. Then, it highlights material synthesis of 2D bismuth atomic sheets with emphasis on physical vapor deposition method with accurate layer controllability and process compatibility with CMOS technology. Moreover, it will survey latest applications of 2D bismuth in terms of electronic, optic, thermoelectric, spintronic and magnetic nanodevices. 2D bismuth derivatives (Bi–X, X = Sb, Te, Se) will also be mentioned as a promising strategy to further improve device performance. At last, it concludes with a brief summary on the current challenges and future prospects in 2D bismuth and its derivatives for innovative electronics, sensors and other devices compatible with CMOS techniques.

On modified eccentric connectivity index of nanotube

Eccentricity based topological indices have attracted large attraction in the field of chemical graph theory. Eccentricity based topological indices are prominent due to their wide range applications. With the help of eccentricity based topological indices we can predict excellent accuracy rate in certain biological activities of diverse nature as com-pared to other indices. Carbon nanotubes are the cylindrical molecules that consist of rolled up sheets of carbon atoms. In this paper we consider the chemical graph of NAn nanotube and compute an important eccentricity based topological index called Modified eccentric connectivity index.

CMOS Compatible Atomic-Precision Donor Devices

Atomic precision advanced manufacturing (APAM) is a technique for placing dopant atoms with single atomic-lattice site precision on silicon surfaces. APAM devices are created by patterning the placement of dopants into a two-dimensional sheet of phosphorus atoms on the silicon surface, incorporating the dopants into the lattice, and then capping the device in silicon. The dopant atoms are incorporated into the silicon lattice through a surface-activated chemical reaction, as opposed to a thermally activated bulk process, which allows for electrically active doping above the solubility limit for phosphorus in silicon. The two-order of magnitude higher doping levels and two-dimensional nature of the dopant sheet enable new device physics and the potential for new device types with superior performance under specific tasks.Atomic precision fabrication requires atomically clean surfaces. Historically the APAM technique has required temperatures in excess of 1200 °C to prepare device surfaces, which has severely limited the ability to combine APAM with other manufacturing techniques such as CMOS. Recent work by our group and others has shown that the surface preparation process can be achieved at temperatures near 800 °C, greatly improving integration possibilities with CMOS [1,2]. Once fabricated, APAM devices have low thermal budgets to minimize vertical diffusion of the phosphorus dopants out of the non-equilibrium two-dimensional sheet. Temperatures on the order of 450 °C have been shown to be permissible for long periods of time.In this talk we report on the initial development of a CMOS process flow that incorporates APAM device fabrication. Based on the thermal budget and processing constraints, our basic process flow involves building the APAM device between CMOS Front-end-of-line (FEOL) and the CMOS Back-end-of-line (BEOL) steps. This is a natural insertion point based upon both thermal budget limitations and the need to directly access the device silicon to build the APAM device. Borrowing from CMOS nomenclature, we introduce the concept of “APAM select” and “APAM active” to indicate where the window for APAM device patterning will be opened and the actual APAM device area, respectively. These are the only additional mask layers needed to introduce APAM into a CMOS layout cell. An APAM cell was created that has the appropriate source/drain placement and targeting reticle in poly-Si to enable alignment of the APAM lithography tool to the wafer.Two classes of challenges exist in developing a CMOS-APAM process flow. The first is related to the APAM process itself and limitations of current fabrication tools. The APAM surface preparation, patterning, dopant incorporation, and silicon capping all occur in a scanning tunneling microscope (STM). Devices must be processed at the die level to fit into the STM sample holders. Die level processing puts restrictions on the BEOL processing including minimum feature size and available processes, such as a lack of chemical-mechanical polish (CMP) to planarize metal layers. The second class of challenges is related to the impacts of the APAM process, such as surface preparation, on CMOS devices, which has not been previously explored. We discuss our strategy for integration with CMOS, by including test structures to mimic the APAM device, and sequential implementation and testing of the APAM fabrication steps with CMOS chips.We conclude with a discussion of future integration challenges such as wafer level processing of APAM devices in a CMOS foundry flow. Direct integration of APAM components into CMOS circuits opens the door for devices with enhanced functionality.ACKNOWLEDGMENTThis work is supported by Sandia’s Lab Directed Research and Development Program, and was performed in part at the Center for Integrated Nanotechnologies, a U.S. DOE Office of Basic Energy Sciences user facility. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

(Invited) Multi-Level Non-Volatile Memory in Au/Monolayer MoS2/Au Structures

The wide range of electronic and optical properties exhibited by two dimensional semiconductors, and their variation physical thickness, opens up new possibilities to create ultra-short length transistors [1], steep-slope switches [2, 3], light detecting/emitting devices [4, 5] and chemical/physical sensors. Recently, it was demonstrated that monolayer 2D semiconductors can also display resistive switching [6]. In this work we demonstrate that a single layer of large area CVD-grown moly disulphide (MoS2) sandwiched between two metal electrodes can be tuned to exhibit multi-level non-volatile resistive memory, with resistance values separated by 5 orders of magnitude. The switching process is unipolar and thermochemically driven (TCM memory) requiring significant Joule heating in the RESET process. Temperature dependent electrical measurements will be presented which demonstrate the charge transport mechanism varies between the initial state (IS), the high resistive state (HRS) and low resistive state (LRS). The temperature dependent behavior of the SET, RESET process is consistent with the creation and passivation of sulfur vacancies in the MoS2 monolayer. Conductive AFM measurements confirm that the multi-filamentary resistive memory effects are inherent to a single crystalline MoS2 triangle and not necessarily dependent of grain boundaries. This work demonstrates that potential for 2 terminal, multi-state, resistive memories for next generation digital memory and neuromorphic applications.[1] S. B. Desai et al., "MoS2 transistors with 1-nanometer gate lengths," Science, vol. 354, no. 6308, pp. 99-102, 2016.[2] S. Bhattacharjee, K. L. Ganapathi, S. Mohan, and N. Bhat, "A sub-thermionic MoS2 FET with tunable transport," Applied Physics Letters, vol. 111, no. 16, p. 163501, 2017.[3] D. Sarkar et al., "A subthermionic tunnel field-effect transistor with an atomically thin channel," Nature, vol. 526, no. 7571, p. 91, 2015.[4] K. Roy et al., "Graphene–MoS 2 hybrid structures for multifunctional photoresponsive memory devices," Nature nanotechnology, vol. 8, no. 11, p. 826, 2013.[5] X. Zhou et al., "2D Layered Material‐Based van der Waals Heterostructures for Optoelectronics," Advanced Functional Materials, vol. 28, no. 14, p. 1706587, 2018.[6] R. Ge et al., "Atomristor: Nonvolatile Resistance Switching in Atomic Sheets of Transition Metal Dichalcogenides," Nano letters, vol. 18, no. 1, pp. 434-441, 2017.

Exciton-driven giant nonlinear overtone signals from buckled hexagonal monolayer GaAs

We report here a giant $\left|\chi_{baa}^{2}\right|=$780 pm/V second harmonic and $\left|\chi_{aaaa}^{3}\right|=$1.4$\times$10$^{-17}$ m$^{2}$/V$^{2}$ third harmonic signal from single atomic sheet of buckled hexagonal GaAs. We demonstrate this through the solution of an ab-initio real-time Bethe-Salpeter equation by including the electron-hole screened-exchange self-energy. The coupling between time-dependent external electric field and correlated electrons is treated within the modern theory of polarization. The result of our calculation envisage monolayer GaAs to be a prominent member in the material library of non-linear signal generations.

Segregation of metallic germanium atoms at the graphene/metal interface toward germanene growth

Germanene, a graphene-like single atomic sheet of germanium (Ge), has attracted much attention due to its exotic properties such as tunable bandgaps and high carrier mobility. However, fabrication of germanene-based electronic devices is difficult due to its chemical instability. To overcome this, we aim to grow germanene at the graphene/metal interface so that the germanene can be protected from oxidation in air. Here, we study Ge segregation in a stacked thin film of graphene/gold (Au)/silver (Ag)/Ge on a silicon substrate to grow germanene. The role of Ag is to suppress Ge segregation at room temperature. In situ X-ray photoelectron spectroscopy revealed that Au/Ag/Ge became an Au–Ag–Ge alloy and metallic Ge atoms were segregated at the graphene/alloy interface by simple heating. The segregation of Ge was unaffected by the existence of the top graphene. An air exposure test clearly proved that the graphene protects Ge from oxidation in air.

Correlating nuclear morphology and external force with combined atomic force microscopy and light sheet imaging separates roles of chromatin and lamin A/C in nuclear mechanics.

Nuclei are often under external stress, be it during migration through tight constrictions or compressive pressure by the actin cap, and the mechanical properties of nuclei govern their subsequent deformations. Both altered mechanical properties of nuclei and abnormal nuclear morphologies are hallmarks of a variety of disease states. Little work, however, has been done to link specific changes in nuclear shape to external forces. Here, we utilize a combined atomic force microscope and light sheet microscope to show SKOV3 nuclei exhibit a two-regime force response that correlates with changes in nuclear volume and surface area, allowing us to develop an empirical model of nuclear deformation. Our technique further decouples the roles of chromatin and lamin A/C in compression, showing they separately resist changes in nuclear volume and surface area, respectively; this insight was not previously accessible by Hertzian analysis. A two-material finite element model supports our conclusions. We also observed that chromatin decompaction leads to lower nuclear curvature under compression, which is important for maintaining nuclear compartmentalization and function. The demonstrated link between specific types of nuclear morphological change and applied force will allow researchers to better understand the stress on nuclei throughout various biological processes.

Quantum Hall Effect in Trilayer Graphene

Graphene is a two-dimensional sheet of carbon atoms with properties that are superior to other materials. Due to these superior properties, a lot of research is being done with this material. One of those properties is the Quantum Hall Effect (QHE). Monolayer graphene shows very interesting behaviour in the presence of a high magneticfield (up to 30T) and at very low temperature (~0.2K). Its unconventional Landau Level (LL) spectrum of massless Diracfermions leads to a new type of quantum Hall effect, known as half-integer quantum Hall effect or anomalous QHE. We expect uniqueresults can also be seen in trilayer graphene (TLG).In this work, we study the integer quantum Hall effect in trilayer graphene theoretically.

A Side-view on Nuclear Mechanics: Combined Atomic Force Microscopy and Light Sheet Microscopy Inform Chromatin's Role in Regulating Nuclear Morphology

A Side-view on Nuclear Mechanics: Combined Atomic Force Microscopy and Light Sheet Microscopy Inform Chromatin's Role in Regulating Nuclear Morphology

The Casimir-Polder interaction of an atom and real graphene sheet: Verification of the Nernst heat theorem

We find the low-temperature behavior of the Casimir-Polder free energy and entropy for an atom interacting with real graphene sheet possessing nonzero energy gap and chemical potential. Employing the formalism of the polarization tensor, it is shown that the Casimir-Polder entropy goes to zero by the power law with vanishing temperature, i.e., the Nernst heat theorem is satisfied. This result is discussed in connection with the problems connected with account of free charge carriers in the Lifshitz theory.

Efficiency and stability enhancement of perovskite solar cells using reduced graphene oxide derived from earth-abundant natural graphite.

Graphene – two-dimensional (2D) sheets of carbon atoms linked in a honeycomb pattern – has unique properties that exhibit great promise for various applications including solar cells. Herein we prepared two-dimensional (2D) reduced graphene oxide (rGO) nanosheets from naturally abundant graphite flakes (obtained from Tuv aimag in Mongolia) using solution processed chemical oxidation and thermal reduction methods. As a proof of concept, we used our rGO as a hole transporting material (HTM) in perovskite solar cells (PSCs). Promisingly, the use of rGO in the hole transporting layer (HTL) not only enhanced the photovoltaic efficiency of PSCs, but also improved the device stability. In particular, the best performing PSC employing rGO nanosheets exhibited a power conversion efficiency (PCE) of up to 18.13%, while the control device without rGO delivered a maximum efficiency of 17.26%. The present work demonstrates the possibilities for solving PSC issues (stability) using nanomaterials derived from naturally abundant graphite sources.

Correlating nuclear morphology and external force with combined atomic force microscopy and light sheet imaging separates roles of chromatin and lamin A/C in nuclear mechanics

Correlating nuclear morphology and external force with combined atomic force microscopy and light sheet imaging separates roles of chromatin and lamin A/C in nuclear mechanics

Preparation of Transparent Conductive Electrode via Layer-By-Layer Deposition of Silver Nanowires and Its Application in Organic Photovoltaic Device.

Solution processed transparent conductive electrodes (TCEs) were fabricated via layer-by-layer (LBL) deposition of silver nanowires (AgNWs). First, the AgNWs were coated on (3-Mercaptopropyl)trimethoxysilane modified glass substrates. Then, multilayer AgNW films were obtained by using 1,3-propanedithiol as a linker via LBL deposition, which made it possible to control the optical transmittance and sheet resistance of multilayer thin films. Next, thermal annealing of AgNW films was performed in order to agent their electrical conductivity. AgNW monolayer films were characterized by UV-Vis spectrometer, field emission scanning electron microscopy, optical microscopy, atomic force microscopy and sheet resistance measurement by four-point probe method. The high performances were achieved with multilayer films, which provided sheet resistances of 9 Ω/sq, 11 Ω/sq with optical transmittances of 71%, 70% at 550 nm, which are comparable to commercial indium tin oxide (ITO) electrodes. Finally, an organic photovoltaic device was fabricated on the AgNW multilayer electrodes for demonstration purpose, which exhibited power conversion efficiency of 1.1%.

Optical and electrical investigations of rubbing assisted few-layer graphene thin film for feasibility of flexible electrode

Transparent conducting electrodes (TCEs) are key components in optoelectronic devices. The commonly used materials for this purpose are indium tin oxide (ITO) and fluorine-doped indium tin oxide (FTO). However, they have limited application for flexible optoelectronic devices due to their brittleness nature. Among various alternatives of TCE materials, graphene has been considered as a good choice owing to its extraordinary properties, such as excellent optical transparency, electrical conductivity mechanical strength along with layer stability on various substrates. In the present work, few-layer graphene (FLG) films have been deposited on flexible polyethylene terephthalate (PET) sheets by rubbing technique at room temperature. The prepared samples were characterized using Optical transparency, Raman spectroscopy, Atomic force microscopy, bending test, and sheet resistance measurements to optimize rubbing parameters. The fabricated FLG films on PET substrate exhibited a sheet resistance 400 Ω/sq. with transmittance ∼60 % indicating its promising application as a flexible transparent conducting electrode in optoelectronic devices.

Extraordinary quasi-two-dimensional magnetotransport properties of a LaAlO3/SrTiO3 heterostructure tailored with a surface TiO2 atomic sheet

Epitaxial heterostructures of lanthanum aluminate (LaAlO3) and strontium titanate (SrTiO3) exhibit extraordinary quasi-two-dimensional magnetotransport properties at low temperatures. To elucidate the mechanisms responsible for the unique properties of these interfaces, which can guide the design of novel structures having high performances, extensive analyses of the magnetotransport properties at low temperatures are required. We report the magnetotransport properties of a LaAlO3/SrTiO3 system tailored with a topmost surface atomic sheet of titanium dioxide (TiO2). Three unit cells of LaAlO3 were deposited on a reconstructed SrTiO3(001)-(13×13)-R33.7° substrate, yielding LaAlO3 covered with a single-atom-thick TiO2 sheet. The high-mobility electrons confined at the LaAlO3/SrTiO3 interface provided significantly high magnetoresistance ratios of +150% and −80% under magnetic fields perpendicular and parallel to the interface, respectively. The in-plane anisotropic magnetoresistance at 4.2 K reached approximately +30%, reflecting the Rashba spin–orbit interactions of the quasi-two-dimensional electrons. A high carrier concentration at the interface realized by the capping of LaAlO3/SrTiO3 with the surface TiO2 sheet significantly contributed to the enhancement of magnetotransport properties arising from the Ti 3d orbitals.

Josephson coupled Ising pairing induced in suspended MoS2 bilayers by double-side ionic gating.

Superconductivity in monolayer transition metal dichalcogenides is characterized by Ising-type pairing induced via a strong Zeeman-type spin-orbit coupling. When two transition metal dichalcogenides layers are coupled, more exotic superconducting phases emerge, which depend on the ratio of Ising-type protection and interlayer coupling strength. Here, we induce superconductivity in suspended MoS2 bilayers and unveil a coupled superconducting state with strong Ising-type spin-orbit coupling. Gating the bilayer symmetrically from both sides by ionic liquid gating varies the interlayer interaction and accesses electronic states with broken local inversion symmetry while maintaining the global inversion symmetry. We observe a strong suppression of the Ising protection that evidences a coupled superconducting state. The symmetric gating scheme not only induces superconductivity in both atomic sheets but also controls the Josephson coupling between the layers, which gives rise to a dimensional crossover in the bilayer.

Graphene deposit in direct current abnormal glow discharge

With the advent of nanotechnology, graphene has gained relevance in the last two decades, due to its exceptional mechanical, electrical and thermal properties. This material being one of the thinnest existing, has at the same time great mechanical strength, flexibility, is transparent, in addition to very good conductor of electricity, much better than copper or any other known material. These properties are due to its structure constituted by a flat monoatomic sheet of carbon atoms arranged cyclically with the same structure of benzene. Several layers (>10) of superimposed graphene constitute the well-known graphite structure. To obtain it, among other methods, plasma enhanced chemical vapor deposition has been used, which uses electric discharges in the radiofrequency regime. In this work, the formation of graphene in the plasma of the abnormal glow discharge of direct current is reported. For this purpose, a mixture of 60% Ar, 5% C2H2 and 35% H2 is used as gas atmosphere, at a pressure of 2.0 torr. Electrolytic copper sheets were used as substrate. Temperature of the process was 600 °C and the formation time was 5 s. Deposits characterization was made by infrared spectroscopy and Raman spectroscopy.

Graphite formation in abnormal glow discharge

Graphite is made up of flat sheets of carbon atoms that form a periodic system of hexagonal rings of six atoms. Along the perpendicular direction of the sheets it has a low electrical conductivity, whereas this is greater throughout them. The possibility of separating the constituent sheets of the graphite and obtaining individual layers of it has led to the current scientific and technological revolution. Graphite can be produced synthetically in the laboratory, with low energy consumption, through the plasma of the abnormal glow discharge in direct current. In this work the abnormal glow discharge graphite formation is reported. The glow was initiated throughout a gaseous atmosphere containing acetylene, utilizing direct current power. In plasma generated in this type of discharge the secondary electrons, coming from the cathode, transfer their kinetic energy to the neutral molecules of the hydrocarbon causing dissociation, ionization and excitation of them. These processes generate active species that react with each other and with the substrate, producing carbon deposits on it. Under controlled conditions of temperature, acetylene content and deposition time, carbon deposits, with the characteristic structure of graphite, in abnormal glow discharge can be obtained. For this purpose, the abnormal glow discharge was established in an atmosphere composed of a mixture of 20% acetylene, 60% argon and 20% hydrogen at low pressure, in a temperature range from 400 °C to 900 °C. Under these conditions, on a copper substrate, a time of 15 minutes was used to obtain the carbon deposits. These deposits were analyzed by infra-red spectroscopy, Raman spectroscopy and scanning electron microscopy, identifying the structure of the graphite and observing the morphology of the deposits on the copper substrate.

Impact of Co on the Ni0.9Co0.1-Ge0.9Sn0.1 Solid-State Reaction and Properties

A systematic and comprehensive study concerning the solid-state reaction between Ni0.9Co0.1 and Ge0.9Sn0.1 layers was carried out. Special attention was paid on the impact of adding 10 at.% of Co in the Ni metallization layer. In-situ X-ray Diffraction (XRD), atomic force microscopy (AFM) and Sheet resistance (Rsh) measurements were performed in order to follow the solid-state reaction, surface morphology and electrical properties evolution, as the temperature increases.

Interlayer Transition in a vdW Heterostructure toward Ultrahigh Detectivity Shortwave Infrared Photodetectors

AbstractVan der Waals (vdW) heterostructures of 2D atomically thin layered materials (2DLMs) provide a unique platform for constructing optoelectronic devices by staking 2D atomic sheets with unprecedented functionality and performance. A particular advantage of these vdW heterostructures is the energy band engineering of 2DLMs to achieve interlayer excitons through type‐II band alignment, enabling spectral range exceeding the cutoff wavelengths of the individual atomic sheets in the 2DLM. Herein, the high performance of GaTe/InSe vdW heterostructures device is reported. Unexpectedly, this GaTe/InSe vdWs p–n junction exhibits extraordinary detectivity in a new shortwave infrared (SWIR) spectrum, which is forbidden by the respective bandgap limits for the constituent GaTe (bandgap of ≈1.70 eV in both the bulk and monolayer) and InSe (bandgap of ≈1.20–1.80 eV depending on thickness reduction from bulk to monolayer). Specifically, the uncooled SWIR detectivity is up to ≈1014 Jones at 1064 nm and ≈1012 Jones at 1550 nm, respectively. This result indicates that the 2DLM vdW heterostructures with type‐II band alignment produce an interlayer exciton transition, and this advantage can offer a viable strategy for devising high‐performance optoelectronics in SWIR or even longer wavelengths beyond the individual limitations of the bandgaps and heteroepitaxy of the constituent atomic layers.