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Physicists obtain molecular 'fingerprints' using plasmons

Posted By Graphene Council, Friday, June 26, 2020
Scientists from the Center for Photonics and 2D Materials of the Moscow Institute of Physics and Technology (MIPT), the University of Oviedo, Donostia International Physics Center, and CIC nanoGUNE have proposed a new way to study the properties of individual organic molecules and nanolayers of molecules. The approach, described in Nanophotonics, relies on V-shaped graphene-metal film structures.

Nondestructive analysis of molecules via infrared spectroscopy is vital in many situations in organic and inorganic chemistry: for controlling gas concentrations, detecting polymer degradation, measuring alcohol content in the blood, etc. However, this simple method is not applicable to small numbers of molecules in a nanovolume. In their recent study, researchers from Russia and Spain propose a way to address this.

A key notion underlying the new technique is that of a plasmon. Broadly defined, it refers to an electron oscillation coupled to an electromagnetic wave. Propagating together, the two can be viewed as a quasiparticle.

The study considered plasmons in a wedge-shaped structure several dozen nanometers in size. One side of the wedge is a one-atom-thick layer of carbon atoms, known as graphene. It accommodates plasmons propagating along the sheet, with oscillating charges in the form of Dirac electrons or holes. The other side of the V-shaped structure is a gold or other electrically conductive metal film that runs nearly parallel to the graphene sheet. The space in between is filled with a tapering layer of dielectric material -- for example, boron nitride -- that is 2 nanometers thick at its narrowest (fig. 1).

Such a setup enables plasmon localization, or focusing. This refers to a process that converts regular plasmons into shorter-wavelength ones, called acoustic. As a plasmon propagates along graphene, its field is forced into progressively smaller spaces in the tapering wedge. As a result, the wavelength becomes many times smaller and the field amplitude in the region between the metal and graphene gets amplified. In that manner, a regular plasmon gradually transforms into an acoustic one.

"It was previously known that polaritons and wave modes undergo such compression in tapering waveguides. We set out to examine this process specifically for graphene, but then went on to consider the possible applications of the graphene-metal system in terms of producing molecular spectra," said paper co-author Kirill Voronin from the MIPT Laboratory of Nanooptics and Plasmonics.

The team tested its idea on a molecule known as CBP, which is used in pharmaceutics and organic light emitting diodes. It is characterized by a prominent absorption peak at a wavelength of 6.9 micrometers. The study looked at the response of a layer of molecules, which was placed in the thin part of the wedge, between the metal and graphene. The molecular layer was as thin as 2 nanometers, or three orders of magnitude smaller than the wavelength of the laser exciting plasmons. Measuring such a low absorption of the molecules would be impossible using conventional spectroscopy.

In the setup proposed by the physicists, however, the field is localized in a much tighter space, enabling the team to focus on the sample so well as to register a response from several molecules or even a single large molecule such as DNA.

There are different ways to excite plasmons in graphene. The most efficient technique relies on a scattering-type scanning near-field microscope. Its needle is positioned close to graphene and irradiated with a focused light beam. Since the needle point is very small, it can excite waves with a very large wave vector -- and a small wavelength. Plasmons excited away from the tapered end of the wedge travel along graphene toward the molecules that are to be analyzed. After interacting with the molecules, the plasmons are reflected at the tapered end of the wedge and then scattered by the same needle that initially excited them, which thus doubles as a detector.

"We calculated the reflection coefficient, that is, the ratio of the reflected plasmon intensity to the intensity of the original laser radiation. The reflection coefficient clearly depends on frequency, and the maximum frequency coincides with the absorption peak of the molecules. It becomes apparent that the absorption is very weak -- about several percent -- in the case of regular graphene plasmons. When it comes to acoustic plasmons, the reflection coefficient is tens of percent lower. This means that the radiation is strongly absorbed in the small layer of molecules," adds the paper's co-author and MIPT visiting professor Alexey Nikitin, a researcher at Donostia International Physics Center, Spain.

After certain improvements to the technological processes involved, the scheme proposed by the Russian and Spanish researchers can be used as the basis for creating actual devices. According to the team, they would mainly be useful for investigating the properties of poorly studied organic compounds and for detecting known ones.

Tags:  2D materials  boron nitride  Graphene  Kirill Voronin  Moscow Institute of Physics and Technology  Photonics  University of Oviedo 

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Defective graphene has high electrocatalytic activity

Posted By Graphene Council, Tuesday, May 12, 2020
Scientists from the Moscow Institute of Physics and Technology, Skoltech, and the Russian Academy of Sciences Joint Institute for High Temperatures have conducted a theoretical study of the effects of defects in graphene on electron transfer at the graphene-solution interface. Their calculations show that defects can increase the charge transfer rate by an order of magnitude. Moreover, by varying the type of defect, it is possible to selectively catalyze the electron transfer to a certain class of reagents in solution. This can be very useful for creating efficient electrochemical sensors and electrocatalysts. The findings were published in Electrochimica Acta.

Carbon is widely used in electrochemistry. A new type of carbon-based electrodes, made of graphene, has great potential for biosensors, photovoltaics, and electrochemical cells. For example, chemically modified graphene can be used as a cheap and effective analogue of platinum or iridium catalysts in fuel cells and metal-air batteries.

The electrochemical characteristics of graphene strongly depend on its chemical structure and electronic properties, which have a significant impact on the kinetics of redox processes. The interest in studying the kinetics of heterogeneous electron transfer on the graphene surface has recently been stimulated by new experimental data showing the possibility of accelerating the transfer at structural defects, such as vacancies, graphene edges, impurity heteroatoms, and oxygen-containing functional groups.

A recent paper co-authored by three Russian scientists presents a theoretical study of the kinetics of electron transfer on the surface of graphene with various defects: single and double vacancies, the Stone-Wales defect, nitrogen impurities, epoxy and hydroxyl groups. All these changes significantly affected the transfer rate constant. The most pronounced effect was associated with a single vacancy: The transfer rate was predicted to grow by an order of magnitude relative to defect-free graphene (fig. 1). This increase should only be observed for redox processes with a standard potential of ?0.2 volts to 0.3 volts -- relative to the standard hydrogen electrode. The calculations also showed that due to the low quantum capacitance of the graphene sheet, the electron transfer kinetics can be controlled by changing the capacitance of the bilayer.

"In our calculations, we tried to establish a relation between the kinetics of heterogeneous electron transfer and the changes in the electronic properties of graphene caused by defects. It turned out that introducing defects into a pristine graphene sheet can lead to an increase in the density of electronic states near the Fermi level and catalyze electron transfer," said Associate Professor Sergey Kislenko of the Department for Physics of High-Temperature Processes, MIPT.

"Also, depending on the kind of defect, it affects the density of electronic states across various energy regions in different ways. This suggests a possibility for implementing selective electrochemical catalysis. We believe that these effects can be useful for electrochemical sensor applications, and the theoretical apparatus that we are developing can be used for targeted chemical design of new materials for electrochemical applications," the scientist added.

Tags:  Graphene  Moscow Institute of Physics and Technology  Sensors  Sergey Kislenko 

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Physicist obtain atomically thin molybdenum disulfide films on large-area substrates

Posted By Graphene Council, Thursday, January 23, 2020
Researchers from the Moscow Institute of Physics and Technology have managed to grow atomically thin films of molybdenum disulfide spanning up to several tens of square centimeters. It was demonstrated that the material's structure can be modified by varying the synthesis temperature. The films, which are of interest to electronics and optoelectronics, were obtained at 900-1,000 degrees Celsius. The findings were published in the journal ACS Applied Nano Materials.

Two-dimensional materials are attracting considerable interest due to their unique properties stemming from their structure and quantum mechanical restrictions. The family of 2D materials includes metals, semimetals, semiconductors, and insulators. Graphene, which is perhaps the most famous 2D material, is a monolayer of carbon atoms. It has the highest charge-carrier mobility recorded to date. However, graphene has no band gap under standard conditions, and that limits its applications.

Unlike graphene, the optimal width of the bandgap in molybdenum disulfide (MoS2) makes it suitable for use in electronic devices. Each MoS2 layer has a sandwich structure, with a layer of molybdenum squeezed between two layers of sulfur atoms. Two-dimensional van der Waals heterostructures, which combine different 2D materials, show great promise as well. In fact, they are already widely used in energy-related applications and catalysis. Wafer-scale (large-area) synthesis of 2D molybdenum disulfide shows the potential for breakthrough advances in the creation of transparent and flexible electronic devices, optical communication for next-generation computers, as well as in other fields of electronics and optoelectronics.

"The method we came up with to synthesize MoS2 involves two steps. First, a film of MoO3 is grown using the atomic layer deposition technique, which offers precise atomic layer thickness and allows conformal coating of all surfaces. And MoO3 can easily be obtained on wafers of up to 300 millimeters in diameter. Next, the film is heat-treated in sulfur vapor. As a result, the oxygen atoms in MoO3 are replaced by sulfur atoms, and MoS2 is formed. We have already learned to grow atomically thin MoS2 films on an area of up to several tens of square centimeters," explains Andrey Markeev, the head of MIPT's Atomic Layer Deposition Lab.

The researchers determined that the structure of the film depends on the sulfurization temperature. The films sulfurized at 500 ? contain crystalline grains, a few nanometers each, embedded in an amorphous matrix. At 700 ?, these crystallites are about 10-20 nm across and the S-Mo-S layers are oriented perpendicular to the surface. As a result, the surface has numerous dangling bonds. Such structure demonstrates high catalytic activity in many reactions, including the hydrogen evolution reaction. For MoS2 to be used in electronics, the S-Mo-S layers have to be parallel to the surface, which is achieved at sulfurization temperatures of 900-1,000 ?. The resulting films are as thin as 1.3 nm, or two molecular layers, and have a commercially significant (i.e., large enough) area.

The MoS2 films synthesized under optimal conditions were introduced into metal-dielectric-semiconductor prototype structures, which are based on ferroelectric hafnium oxide and model a field-effect transistor. The MoS2 film in these structures served as a semiconductor channel. Its conductivity was controlled by switching the polarization direction of the ferroelectric layer. When in contact with MoS2, the La:(HfO2-ZrO2) material, which was earlier developed in the MIPT lab, was found to have a residual polarization of approximately 18 microcoulombs per square centimeter. With a switching endurance of 5 million cycles, it topped the previous world record of 100,000 cycles for silicon channels.

Tags:  2D materials  ACS Applied Nano Materials  Andrey Markeev  Graphene  Moscow Institute of Physics and Technology  Optoelectronics  Semiconductors 

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Graphene Surprises Researchers Again: Strange ‘Melting’ Behavior

Posted By Graphene Council, Monday, January 6, 2020
Physicists from the Moscow Institute of Physics and Technology and the Institute for High Pressure Physics of the Russian Academy of Sciences have used computer modeling to refine the melting curve of graphite that has been studied for over 100 years, with inconsistent findings. They also found that graphene “melting” is in fact sublimation. The results of the study came out in the journal Carbon.

Graphite is a material widely used in various industries — for example in heat shields for spacecraft — so accurate data on its behavior at ultrahigh temperatures is of paramount importance. Graphite melting has been studied since the early 20th century. About 100 experiments have placed the graphite melting point at various temperatures between 3,000 and 7,000 kelvins. With a spread so large, it is unclear which number is true and can be considered the actual melting point of graphite. The values returned by different computer models are also at variance with each other.

A team of physicists from MIPT and HPPI RAS compared several computer models to try and find the matching predictions. Yuri Fomin and Vadim Brazhkin used two methods: classical molecular dynamics and ab initio molecular dynamics. The latter accounts for quantum mechanical effects, making it more accurate. The downside is that it only deals with interactions between a small number of atoms on short time scales. The researchers compared the obtained results with prior experimental and theoretical data.

Fomin and Brazhkin found the existing models to be highly inaccurate. But it turned out that comparing the results produced by different theoretical models and finding overlaps can provide an explanation for the experimental data.

As far back as 1960s, the graphite melting curve was predicted to have a maximum. Its existence points to complex liquid behavior, meaning that the structure of the liquid rapidly changes on heating or densification. The discovery of the maximum was heavily disputed, with a number of studies confirming and challenging it over and over. Fomin and Brazhkin’s results show that the liquid carbon structure undergoes changes above the melting curve of graphene. The maximum therefore has to exist.

The second part of the study is dedicated to studying the melting of graphene. No graphene melting experiments have been conducted. Previously, computer models predicted the melting point of graphene at 4,500 or 4,900 K. Two-dimensional carbon was therefore considered to have the highest melting point in the world.

“In our study, we observed a strange ‘melting’ behavior of graphene, which formed linear chains. We showed that what happens is it transitions from a solid directly into a gaseous state. This process is called sublimation,” commented Associate Professor Yuri Fomin of the Department of General Physics, MIPT. The findings enable a better understanding of phase transitions in low-dimensional materials, which are considered an important component of many technologies currently in development, in fields from electronics to medicine.

The researchers produced a more precise and unified description of how the graphite melting curve behaves, confirming a gradual structural transition in liquid carbon. Their calculations show that the melting temperature of graphene in an argon atmosphere is close to the melting temperature of graphite.

Tags:  2D materials  Graphene  Graphite  Moscow Institute of Physics and Technology  Nanotechnology  Vadim Brazhkin  Yuri Fomin 

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