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.

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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.