Valleytronics in a nutshell

Both classical and quantum computing face significant challenges. On the classical side, silicon field effect transistor technology is reaching the fundamental limits of scaling and there is no replacement technology which has yet demonstrated even comparable performance to the current generation of commercially available silicon CMOS. On the quantum side, scaling the number of entangled superconducting or trapped ion qubits to that required to solve useful problems is an enormous challenge with current device technology. Both fields stand to benefit from transformational devices based on new physical phenomena. Two-dimensional transition metal dichalcogenides (TMDs) possess a number of intriguing electronic, photonic, and excitonic properties.

A lack of inversion symmetry coupled with the presence of time-reversal symmetry endows 2D TMDs with individually addressable valleys in momentum space at the K and K’ points in the first Brillouin zone. This valley addressability opens the possibility of using the momentum state of electrons, holes, or excitons as a completely new paradigm in information processing.

Periodic semiconductor crystal lattices often have degenerate minima in the conduction band at certain points in momentum space. We refer to these minima as valleys, and devices which exploit the fact that carriers are present in one valley versus another are referred to as valleytronic devices. Though degenerate valleys are present in many periodic solids, in most cases it is impossible to address or manipulate carriers in one valley independently from another as the valley state of a carrier is not coupled to any external force we can apply. Thus it is not possible to construct valleytronic devices out of most materials. This is in contrast to spintronics, for example, where the electron spin is readily manipulated by magnetic fields through the electron spin magnetic moment or (less easily) by electric fields through spin-orbit coupling.

In some cases, carrier mass anisotropy along different crystal orientations can result in valley polarization; preferential scattering occurs from one valley into another. This has been shown in diamond, aluminum arsenide, silicon, and bismuth at cryogenic temperatures. However, these materials still lack a strong coupling between the valley index (sometimes called the valley pseudospin) and any external quantity such as an applied field.  It is not clear that there is a way to use mass anisotropy to produce a useful device such as a switch. So we do not consider this class of materials in our discussion of valleytronics.

The recent emergence of 2D materials has provided a more encouraging space in which to explore manipulation and control of the valley index. 2D materials with hexagonal lattices such as graphene or transition metal dichalcogenides (TMDs) can have valleys at the K and K’ points in the Brillouin zone. But to detect or manipulate carriers selectively in one valley we need some measureable physical quantity which distinguishes the two.

The 2H phases of 2D transition metal dichalcogenides lack inversion symmetry and as a result exhibit contrasting Berry curvatures and orbital magnetic moment between the K and K’ valleys. if the Berry curvature has different values at the K and K’ points one can expect different electron, hole, or exciton behavior in each valley as a function of an applied electric field. If the orbital magnetic moment has different values at the K and K’ points one can expect different behavior in each valley as a function of an applied magnetic field. Contrasting values of Berry curvature and orbital magnetic moment at the K and K’ points give rise to optical circular dichroism between the two valleys which allows selective excitation through photons of right or left helicity. Monolayer 2D transition metal dichalcogenides meet this requirement and are the most promising candidates for valleytronic applications.

 

valleytronics
Sketch denoting the circular optical dichroism of the K and -K valley in TMDC monolayers

 

References:

The Valleytronics Materials, Architectures, and Devices Workshop, sponsored by the MIT Linclon Laboratory Technology Office and co-sponsored by NSF, MIT Samberg Center on August 22-23, 2017.

Wu, W. Yao, D. Xiao, T.F. Heinz, “Spin and pseudospins in layered transition metal dichalcogenides”, Nature Physics, 10:343 (2014)

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Life of a PhD Student: Conferences

One of the integral part of being a graduate student is participating in research conferences. Today, I want to change gears from previous blog posts to talk about an important but often undervalued aspect of researcher’s career: attending conferences. I will start with discussing importance of conference for young researchers and later share my personal experience with conferences so far. To begin with, let’s discuss what a typical physics conference entails. Origin of the word conference is Latin word conferre which means ‘bring together’. The formal definition of conference is ‘a formal meeting of people with shared interests’. A scientific conference is an event which generally brings together researcher with all level of expertise and experience working in particular research area or group of areas to present their results and discuss recent advances in the field. This is done usually in a combination of talks and posters by attendees along with more informal coffee and dinner sessions.

 

So that leads us to ask following questions, why do scientists have to attend conferences? Isn’t main goal of our job to work in a lab on something novel and unknown? Isn’t conventional way of publishing paper enough to inform community about the work we are doing? The simplest answer is not really. There are several benefits of attending conferences which makes it essential part of work of a scientist.

 

First of all, dissemination of research through an oral medium like talk or poster is often more effective than simply publishing online or in scientific journal. You are also able to receive feedback on your data/results at various stages of experiment, which can help in guiding project. It also leads you to think about their research with different point of view to present it to broader audience and enables you to improve your communication skills.

 

Attending a conference will also facilitate learning about cutting-edge research in your own research area. It can help you realize how important your current work is for the community and connect it to big-picture research goals. This often leads to new ideas or techniques whose validity you can assess at conference itself and later implement in your own lab.

 

Third benefit is networking. Meeting colleagues working in same field can also help in generating collective ideas or resources and collaborations for future experiments. The importance of collaboration is obvious in present day science, most famous examples are LIGO which led to observation of gravitational waves and CERN led observation of Higgs Boson particle, two of the biggest scientific breakthroughs of 21st century. Another benefit of networking for young researchers is potential to check the fit with different PIs and groups for future positions.

 

Last but not the least, conference gives a huge opportunity to travel around the world, see new culture and make new friends. It gives you a chance to refresh mentally and therefore, it can be considered as fun work time.

 

Our Innovative Training Network Spin-NANO considers scientific conference an important part of training young Early Stage Researchers and plans to organize a conference/meeting every half a year. Most recent meeting was organized at TU Delft in June 2017 along with our Industry Partners. It was a highly engaging 2 days conference where ESRs and project partners were able to introduce, to rest of the network, their work or company respectively. I really enjoyed learning about the industrial culture and challenges from our partners along with scientific results from my fellow ESRs, over talks and conversation over coffee and lunch sessions.

 

The meeting was followed by Think Ahead Workshop where we discussed topics like communicating research to public and presentation skills with immediate feedback on our own presentations during meeting earlier, making us learn about how to better disseminate our research. It was also the first time that all ESRs came together, we had wonderful interaction during the meeting and I am sure it will be start to many wonderful collaborations in next years.

 

Another conference that I attended after starting my PhD was Resonator QED which is organized by Nanosystem Initiative Munich (NIM) in Munich. It is organized every 2 years and is a combination of talks, tutorial talks and poster sessions. It brought together scientists from different fields studying Quantum Electrodynamics (QED) including solid-state cavity QED, atomic cavity QED, circuit QED, single photon sources and quantum memories. This made the entire conference really interesting as scientific goals of these communities are overlapping but system and technology used are widely diverse. While it is not in the scope of blog to summarize entire conference, here I chose to give outline of two of many interesting talks that were presented. Hopefully it will demonstrate exciting work going on in developing quantum technologies using different platforms and also generate your interest to find out more:

 

1) Nanocavity QED: from inverse design to implementation by Prof Vuckovic, Stanford, USA

In her talk, Prof Vuckovic described the work in her group using nanophotonic structures. In one experiment, they have shown strong light-matter coupling between quantum dot (QD) and photonic-crystal cavity creating quasiparticle called polariton and use it to observe dynamic Mollow triplet [1]. In later part of her talk, she discussed an inverse design technique/algorithm to obtain more efficient photonic devices. For example, they have used their algorithm to design compact wavelength demultiplexer (see Figure 1) [2]. This is interesting as it illustrates that the methods from Computer Science are helping physicist to advance integrated photonics.

 

Picture1
Figure 1: Schematic of wavelength demultiplexer with one input and two output waveguide and design region (from ref: [2])
Picture2
Figure 2: Optical micrograph of sample showing superconducting structure and DQD (from ref: [3])

2) Strong coupling of a superconducting resonator to a charge qubit by Prof Ensslin, ETH Zurich, Switzerland

Prof Ensslin, whose group is in fact a member of Spin-Nano network, talked about an experiment that combined solid-state qubit and superconducting quantum interference device (SQUID) array resonator (see Figure 2). SQUID resonator operates in microwave regime and was frequency tunable using magnetic field, it was coupled with GaAs double quantum dot (DQD). They were able to demonstrate strong coupling limit by showing vacuum Rabi mode splitting [3]. This will enable future experiments in quantum information processing using this platform, also known as semiconductor circuit QED.

 

I will conclude by saying that conferences are going to be important part of academic career and one should utilize it fully to their benefit.

 

[1] K. Fischer et al, Nature Photonics 10, 163 (2016) [Web Link]

[2] A. Piggott et al, Nature Photonics 9, 374 (2015) [Web Link]

[3] A. Stockklauser et al, Phys. Rev. X 7, 011030 (2017) [Web Link]

 

By Samarth Vadia, PhD student at attocube and Nanophotonics Group of LMU Munich, Munich, Germany

From the lab into a computer

When I started my PhD in the lovely city of Toulouse I already knew that an increasingly large portion of the solid-state physics community was focusing on the study of 2D materials. Graphene was already well known even outside the scientific community and everyone was describing it as the miraculous material of the future. That was one of the reasons why I started looking for a position in this field.

 

What I’m actually working on different 2D crystals which, contrary to Graphene, act like semiconductors. This family is called Transition Metal Dichalcogenides (TMDs) and in this Spin-NANO blog you will find a lot of details about them: the description of their properties, the possibilities they open up in the creation of new devices or in improving existing ones as well as the challenges in the way. That’s why today I’m not focusing on all of this but I’ll prefer doing something different.

 

I was just checking my newsfeed on Facebook when I saw a post of one of my former colleagues, who just got a permanent position at École Polytechnique in Paris. It was a Nature Communications paper whose title immediately caught my attention: A microprocessor based on a two-dimensional semiconductor. I already knew that in 2011 a MoS2  monolayer based Field Effect Transistor (FET) was built and proven to be operational. However, this paper appeared to talk about a more complex and integrated device, a full microprocessor. I read it as soon as I could.

 

Unfortunately, because of my limited knowledge of microprocessor logic and operation, I didn’t grasp every detail about the device but I was still fascinated by the overall message. The team at the Institute of Photonics at Vienna University of Technology replaced the silicon in the FETs channel with MoS2 bilayer achieving a microprocessor made of 115 transistors which is able to execute user-defined programs, perform operations and communicate the result to outer devices.

 

Microprocessor.png
Microscope image of the TMD transistor microprocessor.

 

One intriguing property of this device is the fact that the substrate can be bendable, opening up the possibility of having flexible electronic devices. However, the most obvious advantage of replacing silicon in transistors with 2D crystals comes from the better geometric scaling and less power consumption that these materials will provide. Which ultimately results in smaller devices with long lasting batteries!

 

Obviously, this prototype microprocessor is still far from the performances of its commercially available counterparts but this is not diminishing my interest on this result as it represents a proof of concept. These devices are doable and making them even more efficient than the ones which are available is just a technological challenge and thus just a question of time.

 

I’m talking about this topic because I work on the other side of the research process: the fundamental research. My aim is principally the understanding of the intrinsic properties of these crystals, and this kind of study is mainly performed for a sense of curiosity rather than for reaching an application goal. It is thus easier to lose the connection between what I’m actually studying and why so many people are working around the world are working on these materials. It’s true that in the introduction of every paper on TMDs you can read how many applications will be possible when our control over them will be good enough, but these often appear distant. Reading that paper instead reminded me how close we are to turn those world into reality.

 

By Marco Manca, PhD student at LPCNO laboratory in Toulouse, France

 

Wachter, S. et al. A microprocessor based on a two-dimensional semiconductor. Nat. Commun. 8, 14948 doi: 10.1038/ncomms14948 (2017)

 

Optical innovations: high reflectance mirrors for the James Webb space telescope

When Galileo pointed his telescope to the sky over 400 years ago, revolutionary scientific discoveries has been made and our look to the natural world has been forever changed. Telescopes have since then replaced the naked eye for observing and discovering the universe. In the following centuries, more powerful and complex telescopes have been introduced. In the early 20s, astronomer Edwin Hubble used the largest telescope of his day to observe galaxies beyond our own at the Mt. Wilson Observatory near Pasadena in California. In April 1990, a telescope of his name was launched from Kennedy Space Center in Florida. The Hubble telescope was the first telescope to be launched in space and allowed since its servicing in 1990 to perform 1.3 million observations and provide valuable data for more than 15 000 scientific papers.

 

hst-sm4
Figure 1: photograph of Hubble Space Telescope taken on the fourth servicing mission to the observatory in 2009, Credits to NASA

 

The Hubble telescope will be soon (Spring 2019) replaced by The James Webb Space Telescope (JWST). NASA Goddard space flight Center, the headquarters of this telescope, has built along the way collaboration with the European and Canadian Space agencies, five more aerospace companies and uses test facilities from several NASA agencies in order to bring this telescope to the world.

 

Originally known as the Next Generation Space Telescope, the biggest asset of the James Webb over the Hubble is its ability to operate in the infrared range only, allowing catching photons from the earliest days of the universe (from 13.5 billion years ago).  Figure 1 helps seeing the design and the structure of the James web telescope: it is a three-mirror anastigmatic telescope. Very close to the Hubble, a Cassegrain telescope, the James Webb telescope is composed of a primary mirror with an opening in its middle which gathers the light and bounces it off a secondary mirror in front of it. The secondary mirror will focus the light into the aft optical subsystem, which contains the tertiary mirror and a fine-steering mirror that helps to stabilize the image.

 

labeledspacecraft
Figure 2: JWST’s subsystems, credit to STSci (NASA, Goddard Space flight Center)

 

The honeycomb primary mirror has a diameter of 6.5m (the width of a tennis court) allowing a light collecting area of 25m2, 6 times greater than the Hubble’s. The primary mirror is composed of 18 hexagonal segments. Each segment is around 1.3m wide. The mirror substrate is made out of Beryllium instead of glass in order to reduce the weight of the mirror. Beryllium (Be) is a light weight material with a low thermal expansion coefficient allowing the mirror to hold its shape in cryogenic temperatures. Beryllium is mined in Utah and purified by Brush Wellman in Ohio. It starts as a fine powder pressed to a flat shape. The block is then cut to blanks put together to form a segment and sent to Axsys Technologies. This company provides the final shape of the Beryllium substrate by cutting away most of the back side and leaving a thin rib structure (Figure 3b). Each rib is 1mm thick. This process allows reducing the substrate weight down to 20kg.

 

After the mirror has been shaped, the front surface is perfectly polished and smoothed out. A cryogenic testing is performed by Ball Aerospace and NASA. The substrates are cooled down to 30K in order to ensure that the material will hold its shape in the space. Corrections are made to the mirrors shape. The mirrors are then sent for gold coating. This coating is performed by Quantum Coatings Inc. in Moorestown.  A 100nm thick gold coating is evaporated on the surface of the substrates with 10nm uniformity over 1.5m wide substrate. The gold reflectance is estimated to 99% over a range from 0.8 to 26μm. The gold coating is very pure and soft. In order to protect the surface from scratches and contaminants, a SiO2 coating is applied. Several tests are performed to check the stress, the reflectance and the roughness of the gold coatings. The mirrors undergo another series of cryogenic testings at NASA for holding it shape.

 

Screen Shot 2017-10-30 at 09.57.35
Figure 3:  a. top photograph: a segment of the primary mirror being controlled by NASA Engineers, b: bottom photograph: back of Beryllium substrate.

 

This article gives an overview of all the process steps, challenges and qualifications for large area mirrors destined to space applications. The James Webb telescope has however more technical challenges related to its numerous components such as NIRSpec, the near infrared camera spectrograph and the Mid Infrared instrument (MIRI).

The James Webb telescope will be launched in Spring 2019 on an Arianne A5 vehicle in French Guiana.

 

By Najwa Sidqi, early stage research at Helia Photonics, Edinburgh.

 

References:

[1] James Webb Space Telescope, Goddard Space Flight Center:

https://jwst.nasa.gov/whatsNext.html

 

[2] NASA: Hubble Space telescope

https://www.nasa.gov/mission_pages/hubble/story/index.html

 

[3] Gold Mirror Coatings for James Webb Space Telescope (JWST)

http://www.quantumcoating.com/projects/

 

[4] James Webb Space Telescope Successor TO Hubble

http://www.ball.com/aerospace/programs/jwst

 

2D Heterostructures

One of the most urgent problems of current electronics industry is related to the size of the elements used in the building of the various devices. All our computers, phones, etc are fabricated using complex, engineered structures generally composed by several layers of materials with different properties.

Society demands that these structures meet, among others, two critical criteria:

– On the one hand, they must be small. Laptops, smart phones, etc are the result of the search for more and more compact devices.

– At the same time, they must become more and more powerful, i.e. perform more operations and quicker. That is, we want smaller structures at no expense of the power of the device.

Gordon E. Moore, one of the co-founders of Intel, empirically stated in 1965 that the number of transistors (the basic electronic element these devices are based off of) per unit area would double every year. It is thanks to the efforts of the scientific community and industry that this has been possible, thus achieving our original goal: the miniaturization of devices at almost no expense. Currently, the largest number of transistors in a commercially available single-chip processor is 7.2 billion in 456 mm2, with transistor sizes in the range of tens of nanometres (1 nm = 1 millionth of a mm), which is basically just some tens of atoms.

However, it is possible that things might get too small. Indeed, transistors, as well as any other electronic devices, are based on the principle by which the motion of electrons can be controlled by tuning some energy levels inside the materials involved. When, however, things are very small, we enter the realm of Quantum Mechanics. This is where the phenomenon of tunnelling might occur: electrons going through a material which should be forbidden to them due to a lack of sufficient energy.

As a comparison, imagine the following. In classical physics, if you have a metallic ball and throw it against a wall, usually the metallic ball will bounce back. However, if you throw it with sufficient energy, it might go through the wall. In quantum physics, though, throwing a ball at low energy implies that the ball has also a non-zero chance of traversing the wall, even though this is classically forbidden.

You can therefore now see the problem: if things get too small we cannot control the motion of electrons anymore by using the energetics of the materials, because these electrons will be able to go through them even in those cases where we don’t want them to.

This is where the 2D materials come in. To build a proper semiconductor structure, at least the following elements are needed: a metal, a semiconductor, and an insulator. As it happens, the realm of 2D materials are capable of providing us with all those three elements: graphene acting as a metal, transition metal dichalcogenides (TMDs) acting as semiconductors, and hexagonal boron nitride (hBN) acting as an insulator.

 

2dheterostructure
Representation of different types of 2D materials and how they can be stacked one on top of the other. Nature 499, 419 (2013)

 

These materials have the advantage of being only 1-3 atoms thick, thus reducing the current size limit of electronic elements by a factor of 10.

The combination of these materials require, however, proper engineering to build so-called 2D heterostructures. You can imagine them as placing Lego bricks one on top of the other to achieve complex structures such as quantum light-emitting diodes1 (LEDs which emit single photons; see post of Luca Sortino to know more about this).

Therefore, the arising of the use of 2D heterostructures has opened the door to many new and exciting physics.

 

1. Nat Comun 7, 12978 (2016)

 

By Alejandro Rodriguez, PhD Student of the Quantum Information and Nanoscale Metrology group of Prof. Atature at the University of Cambridge, UK.

Single photon sources

The nature of light has always attracted the interest of human kind. While a great number of theories were formulated in the past, a huge step towards its comprehension followed Plank’s theory of the black body spectrum in the first years of the 20th century. The quantization of the electromagnetic field in a single harmonic oscillator opened up the doors for the formulation of quantum mechanics and a deeper understanding of light since the times of Newton. Now, light may be described uniquely either as a wave or as a particle, but is composed of wavepackets of energy, that possess the particle-wave duality of quantum mechanics, called photons.

By considering the photon emission statistic of common light sources, i.e. the number of photons that can be detected in a definite time interval, it is interesting to notice that is quite hard to detect single photon emission in nature, in fact such emitters are exclusively a trait of quantum systems.

The reason is that thermal light sources, such as common lamps and the sun, are an ensemble of independent atoms that emit photons at random times, with huge fluctuation in the overall intensity. Due to the bosonic nature of photons, the emission statistic for each mode follows the Bose-Einstein distribution, and the most populated state on average is that of 0 photons, so that no detection for small time interval occurs. Even coherent light, like lasers, that has reduced fluctuation in intensity, shows a Poissonian emission statistic. Hence, no classical system can reproduce the sub-Poissonian emission statistics of a single photon sources, which would ideally always give a single value peaked on 1 photon, as shown in the image below.

photonstatistcs.png
Photon emission statistic of different light sources, m indicates the number of photon impinging on the detector on an arbitrary time interval, while p(m) is the probability of detection.

As stated before, only a two-level quantum system can exhibit a single photon emission. This is due to the fact that, in order to have the emission of a photon from an atom, an electron needs to acquire some energy to jump from the ground state of the system to an excited state; the consequent decay back to the lower energy state is accompanied by the emission of a photon, with energy equal to the difference of the two states. Now that the electron is back again on his fundamental state, we need to excite it again before the subsequent photon emission can be detected. This will induce a measurable dead time between subsequent emissions, proving the single photon emission.

The reason why a lot of interest is given to these emitters is that the possibility of harnessing a single photon state, creating states of light with a huge reduction in noise and a higher degree of coherence, would bring advances in many fields. Due to the fact that information can be encoded in the physical properties of the photon, quantum information processing could rely of these systems like a classical computer relies on transistor to process information. Moreover, advances in secure communication through Quantum Key Distribution protocols, along with an increase in sensing and measurements sensibility, could be achieved.

Interestingly, every atom around us can be seen as a two-level quantum mechanical system, to achieve the control of a single photon emission from one of them we would simply need to isolate that atom from any kind of disturbance and also being able to control their emission properties. While the idea of manipulating such a tiny piece of matter would seem impossible to achieve, the first realisation of single photon states was obtained already in the late 1970s using of attenuated laser beams. Only after the year 2000, new and reliable emitters were developed in solid state systems, such as self-assembled quantum dots (QDs), that acquire an atomic electronic structure due to the three-dimensional confinement, and defects in large band gap crystals [1].

Untitled
Solid state single photon sources.

Since then a multitude of researchers have undertaken studies on various systems in order to pursue a pure single photon emitter and many members of our Spin-NANO community are actively working on solid state emitters like QDs and NV vacancies in diamonds. Some of them, including me, are researching newly discovered solid state single photon emitters in two-dimensional materials, like Transition Metal Dichalcogenides and hexagonal Boron Nitride. Although the first reports of these emitters happened only in the last two years, and they still lack a solid understanding of their nature, the appealing properties of two-dimensional materials combined with the desired qualities of single photon sources hold much promise for future technologies.

 

By Luca Sortino, PhD Student of the Low Dimensional Structures and Devices Group at Univeristy of Sheffield, UK.

[1] Aharonovich, I., Englund, D. & Toth, M. Solid-state single-photon emitters. Nat. Photonics 10, 631–641 (2016) doi:10.1038/nphoton.2016.186

Strained TMD Monolayers: Next Generation Circuitry

Transition metal dichalcogenides (TMD) monolayers, are the worlds thinnest semiconductors. At just three atoms thick, this relatively new breed of materials has captured the attention of the low dimensional materials research community both theoretically and experimentally.

Unlike graphene, a direct band gap in the optical rage can be found at the K and K’ valleys. Not only are these near 2-dimensional materials semiconductors, the cornerstone material upon which a lot of modern technology rests, but they are also particularly desirable semiconductors, with intriguing optical response.

A number of applications have been proposed and demonstrated for TMD monolayers including electrically tuneable mirrors, qubits and single photon emitters. It is in the pursuit of creating single photon emitters in TMD monolayers that the notion of strain engineering has climbed to prominence.

Single photon emitters (SPEs), ideally should deterministically create only 1 photon at a specified time, place and wavelength. This is incredibly difficult to achieve and a number of various methods have been tried over the years. In TMDs, single photon emission from the material has been shown by optically exciting the monolayer either resonantly or above the band gap.

The problem with demonstrations of this effect in TMDs has been the seemingly random nature of the locations of the SPEs. SPEs tend to be located at local band gap minima in K-space, where some potential tends to localise the excitons before the recombine, emitting their photon. In TMDs, these tend to occur in real space around lattice impurities or at the edge of the TMD flakes. At this point in the isolation techniques of TMD monolayers, lattice impurities are to be expected. The SPEs at the edge of the flakes, however, indicates that lattice strain may induce sufficient potential for useful applications.

In this group, one of our colleagues (Alejandro Rodriguez) has shown that this principal may be exploited, creating strain potentials in WS2 and WSe2 flakes by tenting the monolayer about glass nanopillars, showing SPE locations at the top of the pillar.

This principal, of strain induced bandgap renormalisation in TMDs, is my current research interest. I am currently working on developing a theoretical framework which may be used to usefully describe the results of my colleague’s experiment. This, can be done by a combination of the plate theory of continuum mechanics and a tight binding approach to TMD curvature, with parameters from previous DFT strain studies of the materials.

Once a reasonable description of the nanopillar SPE is developed, I hope to see how far this principal can be extended. Can bending the monolayers be used to confine electron in strain induced dots? Can this be used for quantum information? Can wires in the monolayer be defined this way? What are the possible spintronic applications of such wires? Can a similarly large effect be achieved with sagged monolayers? Can this allow for printable TMD circuitry by etching the substrate?

I believe, and hope to soon show, that strain engineering will truly add a new dimension to TMD applications.

adma201600560-fig-0001
Kern, Johannes, et al.’s alternative and nicely illustrated alternative approach to strain engineering, sagging the monolayer over gold nano-bars.

By Matthew Brooks, PhD Student of the Burkard Group at Univeristät Konstanz, Germany.

Kern, Johannes, et al. “Nanoscale Positioning of Single‐Photon Emitters in Atomically Thin WSe2.” Advanced Materials 28.33 (2016): 7101-7105.