1
|
Sarwat SG, Le Gallo M, Bruce RL, Brew K, Kersting B, Jonnalagadda VP, Ok I, Saulnier N, BrightSky M, Sebastian A. Mechanism and Impact of Bipolar Current Voltage Asymmetry in Computational Phase-Change Memory. Adv Mater 2023; 35:e2201238. [PMID: 35570382 DOI: 10.1002/adma.202201238] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/07/2022] [Revised: 03/20/2022] [Indexed: 06/15/2023]
Abstract
Nanoscale resistive memory devices are being explored for neuromorphic and in-memory computing. However, non-ideal device characteristics of read noise and resistance drift pose significant challenges to the achievable computational precision. Here, it is shown that there is an additional non-ideality that can impact computational precision, namely the bias-polarity-dependent current flow. Using phase-change memory (PCM) as a model system, it is shown that this "current-voltage" non-ideality arises both from the material and geometrical properties of the devices. Further, we discuss the detrimental effects of such bipolar asymmetry on in-memory matrix-vector multiply (MVM) operations and provide a scheme to compensate for it.
Collapse
Affiliation(s)
| | - Manuel Le Gallo
- IBM Research-Europe, Säumerstrasse 4, Rüschlikon, 8803, Switzerland
| | - Robert L Bruce
- IBM Research-Yorktown Heights, Yorktown Heights, NY, 10598, USA
| | - Kevin Brew
- IBM Research AI Hardware Center-Albany, Albany, NY, 12203, USA
| | | | | | - Injo Ok
- IBM Research AI Hardware Center-Albany, Albany, NY, 12203, USA
| | - Nicole Saulnier
- IBM Research AI Hardware Center-Albany, Albany, NY, 12203, USA
| | | | - Abu Sebastian
- IBM Research-Europe, Säumerstrasse 4, Rüschlikon, 8803, Switzerland
| |
Collapse
|
2
|
Ghazi Sarwat S, Brückerhoff-Plückelmann F, Carrillo SGC, Gemo E, Feldmann J, Bhaskaran H, Wright CD, Pernice WHP, Sebastian A. An integrated photonics engine for unsupervised correlation detection. Sci Adv 2022; 8:eabn3243. [PMID: 35648858 DOI: 10.1126/sciadv.abn3243] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
With more and more aspects of modern life and scientific tools becoming digitized, the amount of data being generated is growing exponentially. Fast and efficient statistical processing, such as identifying correlations in big datasets, is therefore becoming increasingly important, and this, on account of the various compute bottlenecks in modern digital machines, has necessitated new computational paradigms. Here, we demonstrate one such novel paradigm, via the development of an integrated phase-change photonics engine. The computational memory engine exploits the accumulative property of Ge2Sb2Te5 phase-change cells and wavelength division multiplexing property of optics in delivering fully parallelized and colocated temporal correlation detection computations. We investigate this property and present an experimental demonstration of identifying real-time correlations in data streams on the social media platform Twitter and high-traffic computing nodes in data centers. Our results demonstrate the use case of high-speed integrated photonics in accelerating statistical analysis methods.
Collapse
Affiliation(s)
| | | | | | - Emanuele Gemo
- Department of Engineering of Engineering, University of Exeter, Exeter EX4 4QF, UK
| | | | - Harish Bhaskaran
- Department of Materials, University of Oxford, Oxford OX26HT, UK
| | - C David Wright
- Department of Engineering of Engineering, University of Exeter, Exeter EX4 4QF, UK
| | - Wolfram H P Pernice
- Center for Soft Nanoscience, University of Münster, Busso-Peuss-Str. 10, 48149 Münster, Germany
| | - Abu Sebastian
- IBM Research Europe, Säumerstrasse 4, 8803 Rüschlikon, Switzerland
| |
Collapse
|
3
|
Bragaglia V, Jonnalagadda VP, Sousa M, Sarwat SG, Kersting B, Sebastian A. Structural Assessment of Interfaces in Projected Phase-Change Memory. Nanomaterials (Basel) 2022; 12:nano12101702. [PMID: 35630924 PMCID: PMC9147056 DOI: 10.3390/nano12101702] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/05/2022] [Revised: 04/27/2022] [Accepted: 05/09/2022] [Indexed: 12/04/2022]
Abstract
Non-volatile memories based on phase-change materials have gained ground for applications in analog in-memory computing. Nonetheless, non-idealities inherent to the material result in device resistance variations that impair the achievable numerical precision. Projected-type phase-change memory devices reduce these non-idealities. In a projected phase-change memory, the phase-change storage mechanism is decoupled from the information retrieval process by using projection of the phase-change material’s phase configuration onto a projection liner. It has been suggested that the interface resistance between the phase-change material and the projection liner is an important parameter that dictates the efficacy of the projection. In this work, we establish a metrology framework to assess and understand the relevant structural properties of the interfaces in thin films contained in projected memory devices. Using X-ray reflectivity, X-ray diffraction and transmission electron microscopy, we investigate the quality of the interfaces and the layers’ properties. Using demonstrator examples of Sb and Sb2Te3 phase-change materials, new deposition routes as well as stack designs are proposed to enhance the phase-change material to a projection-liner interface and the robustness of material stacks in the devices.
Collapse
|
4
|
Sarwat SG, Kersting B, Moraitis T, Jonnalagadda VP, Sebastian A. Phase-change memtransistive synapses for mixed-plasticity neural computations. Nat Nanotechnol 2022; 17:507-513. [PMID: 35347271 DOI: 10.1038/s41565-022-01095-3] [Citation(s) in RCA: 17] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/23/2021] [Accepted: 01/12/2022] [Indexed: 06/14/2023]
Abstract
In the mammalian nervous system, various synaptic plasticity rules act, either individually or synergistically, over wide-ranging timescales to enable learning and memory formation. Hence, in neuromorphic computing platforms, there is a significant need for artificial synapses that can faithfully express such multi-timescale plasticity mechanisms. Although some plasticity rules have been emulated with elaborate complementary metal oxide semiconductor and memristive circuitry, device-level hardware realizations of long-term and short-term plasticity with tunable dynamics are lacking. Here we introduce a phase-change memtransistive synapse that leverages both the non-volatility of the phase configurations and the volatility of field-effect modulation for implementing tunable plasticities. We show that these mixed-plasticity synapses can enable plasticity rules such as short-term spike-timing-dependent plasticity that helps with the modelling of dynamic environments. Further, we demonstrate the efficacy of the memtransistive synapses in realizing accelerators for Hopfield neural networks for solving combinatorial optimization problems.
Collapse
|
5
|
Sarwat SG, Bhaskaran H. Memristors get the hues. Nat Nanotechnol 2021; 16:746-747. [PMID: 33888886 DOI: 10.1038/s41565-021-00891-7] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Affiliation(s)
| | - Harish Bhaskaran
- Department of Materials, University of Oxford, Oxford, United Kingdom.
| |
Collapse
|
6
|
Kersting B, Ovuka V, Jonnalagadda VP, Sousa M, Bragaglia V, Sarwat SG, Le Gallo M, Salinga M, Sebastian A. State dependence and temporal evolution of resistance in projected phase change memory. Sci Rep 2020; 10:8248. [PMID: 32427898 PMCID: PMC7237438 DOI: 10.1038/s41598-020-64826-3] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2020] [Accepted: 04/22/2020] [Indexed: 12/02/2022] Open
Abstract
Phase change memory (PCM) is being actively explored for in-memory computing and neuromorphic systems. The ability of a PCM device to store a continuum of resistance values can be exploited to realize arithmetic operations such as matrix-vector multiplications or to realize the synaptic efficacy in neural networks. However, the resistance variations arising from structural relaxation, 1/f noise, and changes in ambient temperature pose a key challenge. The recently proposed projected PCM concept helps to mitigate these resistance variations by decoupling the physical mechanism of resistance storage from the information-retrieval process. Even though the device concept has been proven successfully, a comprehensive understanding of the device behavior is still lacking. Here, we develop a device model that captures two key attributes, namely, resistance drift and the state dependence of resistance. The former refers to the temporal evolution of resistance, while the latter refers to the dependence of the device resistance on the phase configuration of the phase change material. The study provides significant insights into the role of interfacial resistance in these devices. The model is experimentally validated on projected PCM devices based on antimony and a metal nitride fabricated in a lateral device geometry and is also used to provide guidelines for material selection and device engineering.
Collapse
Affiliation(s)
- Benedikt Kersting
- IBM Research - Zurich, Säumerstrasse 4, 8803, Rüschlikon, Switzerland.
| | - Vladimir Ovuka
- IBM Research - Zurich, Säumerstrasse 4, 8803, Rüschlikon, Switzerland
| | | | - Marilyne Sousa
- IBM Research - Zurich, Säumerstrasse 4, 8803, Rüschlikon, Switzerland
| | - Valeria Bragaglia
- IBM Research - Zurich, Säumerstrasse 4, 8803, Rüschlikon, Switzerland
| | - Syed Ghazi Sarwat
- IBM Research - Zurich, Säumerstrasse 4, 8803, Rüschlikon, Switzerland
| | - Manuel Le Gallo
- IBM Research - Zurich, Säumerstrasse 4, 8803, Rüschlikon, Switzerland
| | - Martin Salinga
- Institut für Materialphysik; Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Straße 10, 48149, Münster, Germany
| | - Abu Sebastian
- IBM Research - Zurich, Säumerstrasse 4, 8803, Rüschlikon, Switzerland.
| |
Collapse
|
7
|
Soh EJH, Sarwat SG, Mazzotta G, Porter BF, Riede M, Nicholas R, Kim JS, Bhaskaran H. Filamentary High-Resolution Electrical Probes for Nanoengineering. Nano Lett 2020; 20:1067-1073. [PMID: 31904977 DOI: 10.1021/acs.nanolett.9b04302] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Confining electric fields to a nanoscale region is challenging yet crucial for applications such as high-resolution probing of electrical properties of materials and electric-field manipulation of nanoparticles. State-of-the-art techniques involving atomic force microscopy typically have a lateral resolution limit of tens of nanometers due to limitations in the probe geometry and stray electric fields that extend over space. Engineering the probes is the most direct approach to improving this resolution limit. However, current methods to fabricate high-resolution probes, which can effectively confine the electric fields laterally, involve expensive and sophisticated probe manipulation, which has limited the use of this approach. Here, we demonstrate that nanoscale phase switching of configurable thin films on probes can result in high-resolution electrical probes. These configurable coatings can be both germanium-antimony-tellurium (GST) as well as amorphous-carbon, materials known to undergo electric field-induced nonvolatile, yet reversible switching. By forming a localized conductive filament through phase transition, we demonstrate a spatial resolution of electrical field beyond the geometrical limitations of commercial platinum probes (i.e., an improvement of ∼48%). We then utilize these confined electric fields to manipulate nanoparticles with single nanoparticle precision via dielectrophoresis. Our results advance the field of nanomanufacturing and metrology with direct applications for pick and place assembly at the nanoscale.
Collapse
Affiliation(s)
- Eugene J H Soh
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Syed Ghazi Sarwat
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Giulio Mazzotta
- Clarendon Laboratory, Department of Physics , University of Oxford , Oxford OX1 3PU , United Kingdom
| | - Benjamin F Porter
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Moritz Riede
- Clarendon Laboratory, Department of Physics , University of Oxford , Oxford OX1 3PU , United Kingdom
| | - Robin Nicholas
- Clarendon Laboratory, Department of Physics , University of Oxford , Oxford OX1 3PU , United Kingdom
| | - Judy S Kim
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Harish Bhaskaran
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| |
Collapse
|
8
|
Ghazi Sarwat S, Cheng Z, Youngblood N, Sharizal Alias M, Sinha S, Warner J, Bhaskaran H. Strong Opto-Structural Coupling in Low Dimensional GeSe 3 Films. Nano Lett 2019; 19:7377-7384. [PMID: 31442062 DOI: 10.1021/acs.nanolett.9b03039] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Chalcogenide glasses as nanoscale thin films have become leading candidates for several optical and photonic technologies, ranging from reflective displays and filters to photonic memories. Current material systems, however, show strong optical absorption which limits their performance efficiencies and complicates device level integration. Herein, we report sputter deposited thin films of GeSe3, which are low loss and in which the flexible nature of the atomic structure results in thermally activated tunability in the refractive index as well as in the film's physical volume. Such changes, which occur beyond a threshold temperature are observed to be accumulative and directed toward a more equilibrium amorphous state of the film, instead of crystallization. Our results provide insight into a new type of configurability that is based on strong coupling in the material's opto-structural properties. The low optical losses in this material system combined with the tunability in the optical properties in the visible and near-infrared have direct application in higher performing optical coatings and in corrective optics.
Collapse
Affiliation(s)
- Syed Ghazi Sarwat
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Zengguang Cheng
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Nathan Youngblood
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Mohd Sharizal Alias
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Sapna Sinha
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Jamie Warner
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Harish Bhaskaran
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| |
Collapse
|
9
|
Zhou Y, Sarwat SG, Jung GS, Buehler MJ, Bhaskaran H, Warner JH. Grain Boundaries as Electrical Conduction Channels in Polycrystalline Monolayer WS 2. ACS Appl Mater Interfaces 2019; 11:10189-10197. [PMID: 30817114 DOI: 10.1021/acsami.8b21391] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
We show that grain boundaries (GBs) in polycrystalline monolayer WS2 can act as conduction channels with a lower gate onset potential for field-effect transistors made parallel, compared to devices made in pristine areas and perpendicular to GBs. Localized doping at the GB causes photoluminescence quenching and a reduced Schottky barrier with the metal electrodes, resulting in higher conductivity at lower applied bias values. Samples are grown by chemical vapor deposition with large domains of ∼100 μm, enabling numerous devices to be made within single domains, across GBs and at many similar sites across the substrate to reveal similar behaviors. We corroborate our electrical measurements with Kelvin probe microscopy, highlighting the nature of the doping-type in the material to change at the grain boundaries. Molecular dynamics simulations of the GB are used to predict the atomic structure of the dislocations and meandering tilt GB behavior on the nanoscale. These results show that GBs can be used to provide conduction pathways that are different to transport across GBs and in pristine area for potential electronic applications.
Collapse
Affiliation(s)
- Yingqiu Zhou
- Department of Materials , University of Oxford , Parks Road , Oxford OX1 3PH , U.K
| | - Syed Ghazi Sarwat
- Department of Materials , University of Oxford , Parks Road , Oxford OX1 3PH , U.K
| | | | | | - Harish Bhaskaran
- Department of Materials , University of Oxford , Parks Road , Oxford OX1 3PH , U.K
| | - Jamie H Warner
- Department of Materials , University of Oxford , Parks Road , Oxford OX1 3PH , U.K
| |
Collapse
|
10
|
Sarwat SG, Youngblood N, Au YY, Mol JA, Wright CD, Bhaskaran H. Engineering Interface-Dependent Photoconductivity in Ge 2Sb 2Te 5 Nanoscale Devices. ACS Appl Mater Interfaces 2018; 10:44906-44914. [PMID: 30501199 DOI: 10.1021/acsami.8b17602] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Phase-change materials are increasingly being explored for photonics applications, ranging from high-resolution displays to artificial retinas. Surprisingly, our understanding of the underlying mechanism of light-matter interaction in these materials has been limited to photothermal crystallization because of its relevance in applications such as rewritable optical discs. Here, we report a photoconductivity study of nanoscale thin films of phase-change materials. We identify strong photoconductive behavior in phase-change materials, which we show to be a complex interplay of three independent mechanisms: photoconductive, photoinduced crystallization, and photoinduced thermoelectric effects. We find that these effects also congruously contribute to a substantial photovoltaic effect, even in notionally symmetric devices. Notably, we show that device engineering plays a decisive role in determining the dominant mechanism; the contribution of the photothermal effects to the extractable photocurrent can be reduced to <0.4% by varying the electrodes and device geometry. We then show that the contribution of these individual effects to the photoresponse is phase-dependent with the amorphous state being more photoactive than the crystalline state and that a reversible change occurs in the charge transport from thermionic to tunnelling during phase transformation. Finally, we demonstrate photodetectors with an order of magnitude tunability in photodetection responsivity and bandwidth using these materials. Our results provide insights to the photophysics of phase-change materials and highlight their potential in future optoelectronics.
Collapse
Affiliation(s)
- Syed Ghazi Sarwat
- Department of Materials , University of Oxford , Oxford OX1 3PH , U.K
| | - Nathan Youngblood
- Department of Materials , University of Oxford , Oxford OX1 3PH , U.K
| | - Yat-Yin Au
- Department of Engineering , University of Exeter , Exeter EX4 4QF , U.K
| | - Jan A Mol
- Department of Materials , University of Oxford , Oxford OX1 3PH , U.K
| | - C David Wright
- Department of Engineering , University of Exeter , Exeter EX4 4QF , U.K
| | - Harish Bhaskaran
- Department of Materials , University of Oxford , Oxford OX1 3PH , U.K
| |
Collapse
|
11
|
Tweedie MEP, Sheng Y, Sarwat SG, Xu W, Bhaskaran H, Warner JH. Inhomogeneous Strain Release during Bending of WS 2 on Flexible Substrates. ACS Appl Mater Interfaces 2018; 10:39177-39186. [PMID: 30383356 DOI: 10.1021/acsami.8b12707] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Two-dimensional (2D) materials hold great promise in flexible electronics, but the weak van der Waals interlayer bonding may pose a problem during bending, where easy interlayer sliding can occur. Furthermore, thin films of rigid materials are often observed to delaminate from soft substrates during straining. Here, we study the influence of substrate strain on some of the heterostructure configurations we expect to find in devices, composed of three common 2D materials: graphene, tungsten disulfide, and boron nitride. We used photoluminescence (PL) spectroscopy to measure changes in the heterostructures with strain applied in situ. All heterostructures were fabricated directly on polymer substrates, using materials synthesized by chemical vapor deposition. We observed an inhomogeneous release of strain in all structures, leading to a nonrecoverable broadening of the PL peak and shift of the bandgap. This suggests the need for preconditioning devices before service to ensure stable behavior. A gradual time-dependent relaxation of strain between strain cycles was characterized using time-dependent measurements-an effect which could lead to drift of device behavior during operation. Furthermore, possible degradation was assessed by performing the strain and relax the cycle up to 200 times, where we found little further change after the initial shifts had stabilized. These results have important ramifications for devices fabricated from these and other 2D materials, as they suggest extra processing steps and considerations that must be taken to achieve consistent and stable properties.
Collapse
Affiliation(s)
- Martin E P Tweedie
- Department of Materials , University of Oxford , Parks Road , Oxford OX1 3PH , United Kingdom
| | - Yuewen Sheng
- Department of Materials , University of Oxford , Parks Road , Oxford OX1 3PH , United Kingdom
| | - Syed Ghazi Sarwat
- Department of Materials , University of Oxford , Parks Road , Oxford OX1 3PH , United Kingdom
| | - Wenshuo Xu
- Department of Materials , University of Oxford , Parks Road , Oxford OX1 3PH , United Kingdom
| | - Harish Bhaskaran
- Department of Materials , University of Oxford , Parks Road , Oxford OX1 3PH , United Kingdom
| | - Jamie H Warner
- Department of Materials , University of Oxford , Parks Road , Oxford OX1 3PH , United Kingdom
| |
Collapse
|
12
|
Sarwat SG, Tweedie M, Porter BF, Zhou Y, Sheng Y, Mol J, Warner J, Bhaskaran H. Revealing Strain-Induced Effects in Ultrathin Heterostructures at the Nanoscale. Nano Lett 2018; 18:2467-2474. [PMID: 29510053 DOI: 10.1021/acs.nanolett.8b00036] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
Two-dimensional materials are being increasingly studied, particularly for flexible and wearable technologies because of their inherent thickness and flexibility. Crucially, one aspect where our understanding is still limited is on the effect of mechanical strain, not on individual sheets of materials, but when stacked together as heterostructures in devices. In this paper, we demonstrate the use of Kelvin probe microscopy in capturing the influence of uniaxial tensile strain on the band-structures of graphene and WS2 (mono- and multilayered) based heterostructures at high resolution. We report a major advance in strain characterization tools through enabling a single-shot capture of strain defined changes in a heterogeneous system at the nanoscale, overcoming the limitations (materials, resolution, and substrate effects) of existing techniques such as optical spectroscopy. Using this technique, we observe that the work-functions of graphene and WS2 increase as a function of strain, which we attribute to the Fermi level lowering from increased p-doping. We also extract the nature of the interfacial heterojunctions and find that they get strongly modulated from strain. We observe that the strain-enhanced charge transfer with the substrate plays a dominant role, causing the heterostructures to behave differently from two-dimensional materials in their isolated forms.
Collapse
Affiliation(s)
- Syed Ghazi Sarwat
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Martin Tweedie
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Benjamin F Porter
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Yingqiu Zhou
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Yuewen Sheng
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Jan Mol
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Jamie Warner
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| | - Harish Bhaskaran
- Department of Materials , University of Oxford , Oxford OX1 3PH , United Kingdom
| |
Collapse
|
13
|
Xu W, Li S, Zhou S, Lee JK, Wang S, Sarwat SG, Wang X, Bhaskaran H, Pasta M, Warner JH. Large Dendritic Monolayer MoS 2 Grown by Atmospheric Pressure Chemical Vapor Deposition for Electrocatalysis. ACS Appl Mater Interfaces 2018; 10:4630-4639. [PMID: 29360347 DOI: 10.1021/acsami.7b14861] [Citation(s) in RCA: 40] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The edge sites of MoS2 are catalytically active for the hydrogen evolution reaction (HER), and growing monolayer structures that are edge-rich is desirable. Here, we show the production of large-area highly branched MoS2 dendrites on amorphous SiO2/Si substrates using an atmospheric pressure chemical vapor deposition and explore their use in electrocatalysis. By tailoring the substrate construction, the monolayer MoS2 evolves from triangular to dendritic morphology because of the change of growth conditions. The rough edges endow dendritic MoS2 with a fractal dimension down to 1.54. The highly crystalline basal plane and the edge of the dendrites are visualized at atomic resolution using an annular dark field scanning transmission electron microscope. The monolayer dendrites exhibit strong photoluminescence, which is indicative of the direct band gap emission, which is preserved after being transferred. Post-transfer sulfur annealing restores the structural defects and decreases the n-type doping in MoS2 monolayers. The annealed MoS2 dendrites show good and highly durable HER performance on the glassy carbon with a large exchange current density of 32 μA cmgeo-2, demonstrating its viability as an efficient HER catalyst.
Collapse
Affiliation(s)
- Wenshuo Xu
- Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, U.K
| | - Sha Li
- Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, U.K
| | - Si Zhou
- Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, U.K
| | - Ja Kyung Lee
- Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, U.K
| | - Shanshan Wang
- Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, U.K
| | - Syed Ghazi Sarwat
- Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, U.K
| | - Xiaochen Wang
- Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, U.K
| | - Harish Bhaskaran
- Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, U.K
| | - Mauro Pasta
- Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, U.K
| | - Jamie H Warner
- Department of Materials, University of Oxford , Parks Road, Oxford OX1 3PH, U.K
| |
Collapse
|
14
|
Abstract
Graphene nanogap electrodes have been of recent interest in a variety of fields, ranging from molecular electronics to phase change memories. Several recent reports have highlighted that scaling graphene nanogaps to even smaller sizes is a promising route to more efficient and robust molecular and memory devices. Despite the significant interest, the operating and scaling limits of these electrodes are completely unknown. In this paper, we report on our observations of consistent voltage driven resistance switching in sub-5 nm graphene nanogaps. We find that such electrical switching from an insulating state to a conductive state occurs at very low currents and voltages (0.06 μA and 140 mV), independent of the conditions (room ambient, low temperatures, as well as in vacuum), thus portending potential limits to scaling of functional devices with carbon electrodes. We then associate this phenomenon to the formation and rupture of carbon chains. Using a phase change material in the nanogap as a demonstrator device, fabricated using a self-alignment technique, we show that for gap sizes approaching 1 nm the switching is dominated by such carbon chain formation, creating a fundamental scaling limit for potential devices. These findings have important implications, not only for fundamental science, but also in terms of potential applications.
Collapse
Affiliation(s)
- Syed Ghazi Sarwat
- Department of Materials, University of Oxford , Oxford, OX1 3PH, United Kingdom
| | - Pascal Gehring
- Department of Materials, University of Oxford , Oxford, OX1 3PH, United Kingdom
| | | | - Jamie H Warner
- Department of Materials, University of Oxford , Oxford, OX1 3PH, United Kingdom
| | - G Andrew D Briggs
- Department of Materials, University of Oxford , Oxford, OX1 3PH, United Kingdom
| | - Jan A Mol
- Department of Materials, University of Oxford , Oxford, OX1 3PH, United Kingdom
| | - Harish Bhaskaran
- Department of Materials, University of Oxford , Oxford, OX1 3PH, United Kingdom
| |
Collapse
|