Two dimensional III-VI semiconductors and graphene-hybrid heterostructures

Lead Research Organisation: University of Sheffield
Department Name: Physics and Astronomy

Abstract

The isolation of single-atomic layer graphene has led to a surge of interest in other layered crystals with strong in-plane bonds and weak, van der Waals-like, interlayer coupling. A variety of two-dimensional (2D) crystals have been investigated, including large band gap insulators and semiconductors with smaller band gaps such as transition metal dichalcogenides. Interest in these systems is motivated partly by the need to combine them with graphene to create field effect transistors with high on-off switching ratios. More importantly, heterostructures made by stacking different 2D crystals on top of each other provide a platform for creating new artificial crystals with potential for discoveries and applications.
The possibility of making van der Waals heterostructures has been demonstrated experimentally only for a few 2D crystals. However, some of the currently available 2D layers are unstable under ambient conditions, and those that are stable offer only limited functionalities, i.e. low carrier mobility, weak optical emission/absorption, band gaps that cannot be tuned, etc. In a recent series of pilot experiments, we have demonstrated that nanoflakes of the III-VI layer compound, InSe, with thickness between 5 and 20 nanometers, have a "thickness-tuneable" direct energy gap and a sufficiently high chemical stability to allow us to combine them with graphene and related layer compounds to make heterostructures with novel electrical and optical properties. The main goal of this project is to develop graphene-hybrid heterostructures based on this novel class of two-dimensional (2D) III-VI van der Waals crystals. This group of semiconductors will enrich the current "library" of 2D crystals by overcoming limitations of currently available 2D layers and by offering a versatile range of electronic and optical properties. From the growth and fabrication of new systems to the demonstration of prototype devices, including vertical tunnel transistors and optical-enhanced-microcavity LEDs, our project will provide a platform for scientific investigations and will contribute to the technology push required to create new routes to device miniaturization, fast-electronics, sensing and photonics. There is great potential for further growth of all these sectors as the fabrication of 2D systems improves and as new properties are discovered and implemented in functional devices.
 
Description A number of layered 2D semiconducting materials were investigated. We found that generally photon emission efficiency in GaSe and GaTe significantly diminishes for films with small thickness, which makes these materials in their current form (and crystal quality) not suitable for light emitting applications. Further investigations are underway to study these materials in absorption where they may be used as light detectors. A number of substrates suitable for enhancement of absorption is investigated. We also investigated the stability of GaSe and InSe and found that InSe is by far more stable than GaSe.

We have discovered microscopic defect centres in GaSe that work as single photon emitters. In collaboration with University of Munster, we identified important properties of this single photon emitting centres. We revealed the spectral lines corresponding to exciton and biexciton transitions of the quantum emitters with power-dependent photoluminescence and photon statistics measurements. We found evidence that the localization of the excitons is related to deformations of the GaSe crystal, caused by nanoscale selenium inclusions, which are incorporated in the crystal. These deformations give rise to local strain fields, which induce confinement potentials for the excitons. This mechanism lights the way for the controlled positioning of single-photon emitters in GaSe on the nanoscale. In collaboration with Munster we have now investigated coupling of such single photon emitters with on-chip waveguide modes. This work was published in Nano Letter in 2017.

A number of other experiments on a different type of layered semiconductors, transition metal dichalcogenides (TMDs), has been carried out. Notably this includes collaborative work with Prof Manfred Bayer (TU Dortmund), Prof Jeremy Baumberg (Cambridge) and Prof Wolgang Langbein (Cardiff University). This work was possible because Sheffield progressed significantly in fabrication of few atomic layer samples.
Exploitation Route The main outcome of our work is in better understanding of how atomically thin films should be handled, what substrates to use and what protective/capping layers should be applied. In a long term this may be of interest to opto-electronics sector and semiconductor device manufacturing.

Surprisingly, our latest findings open the way for few layer GaSe in quantum applications. In total we published two papers on research in III-VI materials: (1) O. Del Pozo-Zamudio, S. Schwarz, M. Sich, I. A. Akimov, M. Bayer, R. C. Schofield, E. A. Chekhovich, B. J. Robinson, N. D. Kay, O. V. Kolosov, A. I. Dmitriev, G. V. Lashkarev, D. N. Borisenko, N. N. Kolesnikov, A. I. Tartakovskii, "Photoluminescence of two-dimensional GaTe and GaSe films", 2D Materials, 2, 035010 (2015); (2) P. Tonndorf, S. Schwarz, J. Kern, I. Niehues, O. Del Pozo Zamudio, A. Dmitriev, A. Bakhtinov, D. Borisenko, N. Kolesnikov, A. I. Tartakovskii, S. Michaelis de Vasconcellos, R. Bratschitsch, "Single-photon emitters in GaSe", 2D MATERIALS, 4, 2 (2017); (3) P. Tonndorf, O. Del Pozo Zamudio, N. Gruhler, J. Kern, R. Schmidt, A. I. Dmitriev, A. P. Bakhtinov, A. I. Tartakovskii, W. H. P. Pernice, S. Michaelis de Vasconcellos, R. Bratschitsch, "On-chip waveguide coupling of a layered semiconductor single-photon source", NANO LETTERS 17, 5446 (2017); (4) O. Del Pozo-Zamudio, S. Schwarz, J. Klein, R. C. Schofield, E. A. Chekhovich, O. Ceylan, E. Margapoti, A. I. Dmitriev, G. V. Lashkarev, D. N. Borisenko, N. N. Kolesnikov, J. J. Finley, A. I. Tartakovskii, "Photoluminescence and Raman investigation of stability of InSe and GaSe thin films", arXiv:1506.05619 (2015).
Sectors Electronics,Energy,Healthcare,Manufacturing, including Industrial Biotechology

 
Description Engineering and Physical Sciences Research Council (EPSRC), responsive mode
Amount £1,234,905 (GBP)
Funding ID EP/P026850/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 08/2017 
End 07/2020
 
Title Development of a technique for rapid identification of monolayer semiconductors 
Description We developed an efficient method for monitoring interlayer coupling in heterostructures made from transition metal dichalcogenides using photoluminescence imaging in a bright-field optical microscope. The color and brightness in such images are used here to identify mono- and few-layer crystals and to track changes in the interlayer coupling and the emergence of interlayer excitons after thermal annealing in heterobilayers composed of mechanically exfoliated flakes and as a function of the twist angle in atomic layers grown by chemical vapor deposition. Material and crystal thickness sensitivity of the presented imaging technique makes it a powerful tool for characterization of van der Waals heterostructures assembled by a wide variety of methods, using combinations of materials obtained through mechanical or chemical exfoliation and crystal growth. 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? Yes  
Impact The method is now widely used in Sheffield as a characterisation tool for 2D samples. The paper describing this technique is published, and the method may be copied by any other group. Publication: E. Alexeev, A. Catanzaro, O. Skrypka, P. Nayak, S. Ahn, S. Pak, J. Lee, J. I. Sohn; K. Novoselov; H. S. Shin, A. Tartakovskii, "Imaging of interlayer coupling in van der Waals heterostructures using a bright-field optical microscope", NANO LETTERS 17, 5342 (2017). 
URL https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b01763
 
Title Methods for production of two-dimensional semiconducting films and their heterostructures 
Description Methods for production of two-dimensional semiconducting films and their heterostructures have been developed using a dedicated set-up consisting of a microscope and special sample holder. Special processes have been developed enabling heterostructures built from 2D films to be constructed . 
Type Of Material Improvements to research infrastructure 
Year Produced 2016 
Provided To Others? Yes  
Impact Samples have been provided for experiments in Cardiff, Dortmund and Muenster 
 
Description Collaboration with Prof Jeremy Baumberg 
Organisation University of Cambridge
Department Cavendish Laboratory
Country United Kingdom 
Sector Academic/University 
PI Contribution We produced a wide range of few atomic layer semiconductor samples and found a way to deposit them on single-crystal gold supplied by Prof Baumberg's group. We participated in analysis and interpretation of the data and writing the manuscript.
Collaborator Contribution Prof Baumberg came with an idea of using layered semiconductors in ultra-compact plasmonic nano-cavities. They provided single-crystal gold substrates for deposition of layered semiconductors, then deposited gold nanoparticles and conducted all optical measurements.
Impact Published one paper: M.-E. Kleemann, R. Chikkaraddy, E. M. Alexeev, D. Kos, C. Carnegie, W. Deacon, A. Casalis de Pury, C. Große, B. de Nijs, J. Mertens, A. I. Tartakovskii, J. J. Baumberg, "Strong-coupling of WSe2 in ultra-compact plasmonic nanocavities at room temperature", NATURE COMMUNICATIONS 8, 1296 (2017).
Start Year 2016
 
Description Collaboration with Prof Manfred Bayer 
Organisation Technical University of Dortmund
Country Germany 
Sector Academic/University 
PI Contribution We fabricated GaSe and GaTe samples and did initial characterisation. At a later stage we also carried out initial characterisation of MoSe2 and WSe2 samples. We formulated the problem that needed to be addressed in ultra-fast spectroscopy experiments in Dortmund. Researchers from Sheffield visited Dortmund for a total of 6 weeks (3 visits, 2 weeks each) to conduct joint experiments.
Collaborator Contribution Prof Manfred Bayer group are the leading experts in Europe in ultra-fast optical spectroscopy. In these joint experiments they used their streak-camera and various tunable lasers to measure photoluminescence dynamics in Gase, GaTe, MoSe2 and WSe2.
Impact Published 2 papers: T. Godde, D. Schmidt, J. Schmutzler, M. Aßmann, J. Debus, F. Withers, E. M. Alexeev, O. Del Pozo-Zamudio, O. V. Skrypka, K. S. Novoselov, M. Bayer, A. I. Tartakovskii, "Exciton and trion dynamics in atomically thin MoSe2 and WSe2: Effect of localization", PHYSICAL REVIEW B 94, 165301 (2016); O. Del Pozo-Zamudio, S. Schwarz, M. Sich, I. A. Akimov, M. Bayer, R. C. Schofield, E. A. Chekhovich, B. J. Robinson, N. D. Kay, O. V. Kolosov, A. I. Dmitriev, G. V. Lashkarev, D. N. Borisenko, N. N. Kolesnikov, A. I. Tartakovskii, "Photoluminescence of two-dimensional GaTe and GaSe films", 2D Materials, 2, 035010 (2015).
Start Year 2015
 
Description Collaboration with Prof Rudolf Bratschitsch 
Organisation University of Münster
Country Germany 
Sector Academic/University 
PI Contribution Sheffield discovered sharp emission lines in the spectrum of thin films of GaSe. We made initial studies and expected that these lines could behave as single photon emitters. We then passed samples to Prof Rudolf Bratschitsch in Munster, who indeed confirmed that these were single photon emitters. They then used these material to also deposit it on waveguides, and conducted further work showing the coupling of single photon emitters to the waveguide mode. Sheffield contributed with the initial discovery and experiments as well as with expertise at the manuscript writing stages.
Collaborator Contribution Prof Rudolf Bratschitsch's group in Munster confirmed that the sharp lines found by Sheffield in a few layer GaSe were indeed single photon emitters. They then used these material to deposit it on waveguides, and conducted further work showing the coupling of single photon emitters to the waveguide mode.
Impact We published two papers: P. Tonndorf, O. Del Pozo Zamudio, N. Gruhler, J. Kern, R. Schmidt, A. I. Dmitriev, A. P. Bakhtinov, A. I. Tartakovskii, W. H. P. Pernice, S. Michaelis de Vasconcellos, R. Bratschitsch, "On-chip waveguide coupling of a layered semiconductor single-photon source", Nano Letters, 17, 5446 (2017); P. Tonndorf, S. Schwarz, J. Kern, I. Niehues, O. Del Pozo Zamudio, A. Dmitriev, A. Bakhtinov, D. Borisenko, N. Kolesnikov, A. I. Tartakovskii, S. Michaelis de Vasconcellos, R. Bratschitsch, "Single-photon emitters in GaSe", 2D MATERIALS, 4, 2 (2017).
Start Year 2016
 
Description Collaboration with Prof Wolfgang Langbein 
Organisation Cardiff University
Department School of Physics and Astronomy
Country United Kingdom 
Sector Academic/University 
PI Contribution We studied gated structures of MoSe2/hBN and performed their full optical characterisation. We then supplied this samples to Cardiff.
Collaborator Contribution Prof Langbein's group carried out detailed four-wave-mixing investigations of the samples.
Impact One paper was published: L. Scarpelli, F. Masia, E. M. Alexeev, F. Withers, A. I. Tartakovskii, K. S. Novoselov, W. Langbein, "Resonantly excited exciton dynamics in two-dimensional MoSe2 monolayers", PHYSICAL REVIEW B 96, 045407 (2017).
Start Year 2015
 
Description 2D Materials, Youtube video 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact A yuotube video was created. A group of researchers from Sheffield wrote a script and then worked with a Sheffield-based animation company 23i to create a video. This video describes basics of 2D materials and how they can be combined in so-called heterostructures to build novel devices. This video was posted in Oct 2016 and has since been watched 11,500 times.
Year(s) Of Engagement Activity 2016,2017,2018
URL https://www.youtube.com/watch?v=jkAXhJWixJ8