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Antal Koos

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Kansas State University

Viacheslav Kuleshov

Stockholm University

Marianna Charitonidou

Princeton University

Hadia Hassan Selim

National Research Institute Of Astronomy And Geophysics Nriag

Abdel-naby Saad

National Research Institute Of Astronomy And Geophysics Nriag

Harijono Djojodihardjo

Institut Teknologi Bandung

Georgia Zacharopoulou

Hellenic Ministry of Culture

National Council for Scientific Research

Elbaz I . Abouelmagd

National Research Institute Of Astronomy And Geophysics Nriag

SUNY: Stony Brook University

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Papers by Antal Koos

Tailoring MoS2 domains size, doping, and light emission by the sulfurization temperature of ultra-thin MoOx films on sapphire

Applied physics letters, Jun 10, 2024

Large-Area MoS2 Films Grown on Sapphire and GaN Substrates by Pulsed Laser Deposition

Nanomaterials, Oct 25, 2023

In this paper, we present the preparation of few-layer MoS 2 films on single-crystal sapphire, as... more In this paper, we present the preparation of few-layer MoS 2 films on single-crystal sapphire, as well as on heteroepitaxial GaN templates on sapphire substrates, using the pulsed laser deposition (PLD) technique. Detailed structural and chemical characterization of the films were performed using Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction measurements, and high-resolution transmission electron microscopy. According to X-ray diffraction studies, the films exhibit epitaxial growth, indicating a good in-plane alignment. Furthermore, the films demonstrate uniform thickness on large areas, as confirmed by Raman spectroscopy. The lateral electrical current transport of the MoS 2 grown on sapphire was investigated by temperature (T)-dependent sheet resistance and Hall effect measurements, showing a high n-type doping of the semiconducting films (n s from ~1 × 10 13 to ~3.4 × 10 13 cm -2 from T = 300 K to 500 K), with a donor ionization energy of E i = 93 ± 8 meV and a mobility decreasing with T. Finally, the vertical current injection across the MoS 2 /GaN heterojunction was investigated by means of conductive atomic force microscopy, showing the rectifying behavior of the I-V characteristics with a Schottky barrier height of φ B ≈ 0.36 eV. The obtained results pave the way for the scalable application of PLD-grown MoS 2 on GaN in electronics/optoelectronics.

Interface Properties of MoS2 van der Waals Heterojunctions with GaN

Nanomaterials, Jan 4, 2024

Waals Heterojunctions with GaN.

Atomic resolution interface structure and vertical current injection in highly uniform MoS2 heterojunctions with bulk GaN

Applied Surface Science, May 1, 2023

The integration of two-dimensional MoS2 with GaN recently attracted significant interest for futu... more The integration of two-dimensional MoS2 with GaN recently attracted significant interest for future electronic/optoelectronic applications. However, the reported studies have been mainly carried out using heteroepitaxial GaN templates on sapphire substrates, whereas the growth of MoS2 on lowdislocation-density bulk GaN can be strategic for the realization of "truly" vertical devices. In this paper, we report the growth of ultrathin MoS2 films, mostly composed by single-layers (1L), onto homoepitaxial n-GaN on n + bulk substrates by sulfurization of a pre-deposited MoOx film. Highly uniform and conformal coverage of the GaN surface was demonstrated by atomic force microscopy, while very low tensile strain (0.05%) and a significant p +-type doping (4.5×10 12 cm-2) of 1L-MoS2 was evaluated by Raman mapping. Atomic resolution structural and compositional analyses by aberration-corrected electron microscopy revealed a nearly-ideal van der Waals interface between MoS2 and the Ga-terminated GaN crystal, where only the topmost Ga atoms are affected by oxidation. Furthermore, the relevant lattice parameters of the MoS2/GaN heterojunction, such as the van der Waals gap, were measured with high precision. Finally, the vertical current injection across this 2D/3D heterojunction has been investigated by nanoscale current-voltage analyses performed by conductive atomic force microscopy, showing a rectifying behavior with an average turn-on voltage Von=1.7 V under forward bias, consistent with the expected band alignment at the interface between p + doped 1L-MoS2 and n-GaN.

Interface structure and doping of CVD grown MoS<sub>2</sub> on 4H‐SiC by microscopic analyses and <i>ab initio</i> calculations

Physica Status Solidi (rrl), Jul 25, 2023

The interface structure and electronic properties of monolayer (1L) MoS2 domains grown by chemica... more The interface structure and electronic properties of monolayer (1L) MoS2 domains grown by chemical vapor deposition on 4H–SiC(0001) are investigated by microscopic/spectroscopic analyses combined with ab initio calculations. The triangular domains are epitaxially oriented on the (0001) basal plane, with the presence of a van der Waals (vdW) gap between 1L–MoS2 and the SiC terraces. The high crystalline quality of the domains is confirmed by photoluminescence emission. Furthermore, a very low tensile strain (ε ≈ 0.03%) of 1L–MoS2, consistent with the small in‐plane lattice mismatch, and a p‐type doping of (0.45 ± 0.11) × 1013 cm−2, is evaluated by Raman mapping. Density functional theory (DFT) calculations of the MoS2/4H–SiC(0001) system are also performed, considering different levels of refinement of the model: 1) the simple case of the junction between Si‐terminated SiC and MoS2, showing a covalent bond between the Si–S atoms and n‐type doping of MoS2; 2) the complete passivation of Si dangling bonds with a monolayer (1 ML) of oxygen atoms, resulting in a vdW bond with dSi–S ≈ 3.84 Å bond length and p‐type doping of MoS2; and 3) partial (¼ ML and ½ ML) oxygen coverages of the 4H–SiC surface, resulting in intermediate values of dSi–S and doping behavior.

Indium Nitride at the 2D Limit

Advanced Materials, Nov 23, 2020

The properties of 2D InN are predicted to substantially differ from the bulk crystal. The predict... more The properties of 2D InN are predicted to substantially differ from the bulk crystal. The predicted appealing properties relate to strong in‐ and out‐of‐plane excitons, high electron mobility, efficient strain engineering of their electronic and optical properties, and strong application potential in gas sensing. Until now, the realization of 2D InN remained elusive. In this work, the formation of 2D InN and measurements of its bandgap are reported. Bilayer InN is formed between graphene and SiC by an intercalation process in metal–organic chemical vapor deposition (MOCVD). The thickness uniformity of the intercalated structure is investigated by conductive atomic force microscopy (C‐AFM) and the structural properties by atomic resolution transmission electron microscopy (TEM). The coverage of the SiC surface is very high, above 90%, and a major part of the intercalated structure is represented by two sub‐layers of indium (In) bonded to nitrogen (N). Scanning tunneling spectroscopy (STS) measurements give a bandgap value of 2 ± 0.1 eV for the 2D InN. The stabilization of 2D InN with a pragmatic wide bandgap and high lateral uniformity of intercalation is demonstrated.

Interface broadening due to Ar+ ion bombardment measured on Co/Cu multilayer at grazing angle of incidence

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2003

A Co/Cu multilayer system, with double layer thickness of about 60 nm was Auger electron spectros... more A Co/Cu multilayer system, with double layer thickness of about 60 nm was Auger electron spectroscopy depth profiled applying 1 keV Ar+ bombardment, varying the angle of incidence in the range of 78°–86°. It was found that (a) the relative sputter rate changes with the angle of incidence; (b) the interface broadening of the Co/Cu transition is larger than that of the Cu/Co transition and depends on the depth; (c) the broadening of the Co/Cu transition increases with increasing relative sputter rate; and (d) the broadening of the Cu/Co transition does not depend on the relative sputter rate. These features will be explained as the consequences of the strong preferential sputtering of Cu (with respect to Co) in this angular range. Because of the preferential sputtering ion bombardment induced roughening and smoothening (which was directly observed by atomic force microscopy measurements) occurs on the Co/Cu and Cu/Co interface, respectively. D-TRIM simulation has been also performed w...

Fókuszált ionsugaras és elektronsugaras nano-megmunkálás fizikai-kémiai alapjainak vizsgálata különös tekintettel a potenciális nanoelektronikai alkalmazásokra = Investigation of physical and chemical basics of focused electron and ion beam nano-machining with special emphasis on possible applica...

Részletes jelentés Az MTA Műszaki Fizikai és Anyagtudományi Kutatóintézet irányultsága a kétezres... more Részletes jelentés Az MTA Műszaki Fizikai és Anyagtudományi Kutatóintézet irányultsága a kétezres évek elejétől fokozottan fordult a nanoméretek fizikája és technológiája felé. Fontos lépés volt ezen az úton a pásztázó nanosugaras vizsgálati és megmunkálási módszerek "hadrendbe állítása"u.m.-kisenergiás pásztázó elektronmikroszkópia (LVSEM) és mikroanalízis,-mikroszkópia és nanoméretekben folytatott marás és mintázatok kialakítása fókuszált ionsugár (FIB) segítségével az LVSM és a FIB sugarát felhasználva gázfázisú prekurzorok injektálásával (GIS) anyagleválasztás nanoméretekben A LVSEM lehetőségei: milliószoros nagyítás szigetelő Al2O3 szemcséről, és különféle C nanocsövek képei brought to you by CORE View metadata, citation and similar papers at core.ac.uk

Time dependent decomposition of ammonia borane for the controlled production of 2D hexagonal boron nitride

Scientific Reports, Oct 30, 2017

Ammonia borane (AB) is among the most promising precursors for the large-scale synthesis of hexag... more Ammonia borane (AB) is among the most promising precursors for the large-scale synthesis of hexagonal boron nitride (h-BN) by chemical vapour deposition (CVD). Its non-toxic and non-flammable properties make AB particularly attractive for industry. AB decomposition under CVD conditions, however, is complex and hence has hindered tailored h-BN production and its exploitation. To overcome this challenge, we report in-depth decomposition studies of AB under industrially safe growth conditions. In situ mass spectrometry revealed a time and temperature-dependent release of a plethora of N x B y-containing species and, as a result, significant changes of the N:B ratio during h-BN synthesis. Such fluctuations strongly influence the formation and morphology of 2D h-BN. By means of in situ gas monitoring and regulating the precursor temperature over time we achieve uniform release of volatile chemical species over many hours for the first time, paving the way towards the controlled, industrially viable production of h-BN. Hexagonal boron nitride (h-BN) is a layered ceramic material with an atomic structure similar to that of graphite, but with a wide band gap of 5.7 eV 1. Monolayer or few-layer h-BN has received considerable attention recently as a substrate for graphene and other 2D-nanomaterials. Due to the low number of charge impurities in h-BN the mobility of supported graphene improved by more than an order of magnitude 2 compared to graphene on the commonly used silicon dioxide. Highly crystalline h-BN domains serve as a better template for the epitaxial growth of monolayer graphene 3 and WS 2 4 , showing good alignment of the domains. The possibility of forming heterostructures wholly from the 2D family of nanomaterials opens many exciting opportunities in the electronics industry. Radio frequency electronics 5 , nanocapacitors 6 , flexible and transparent transistors 7 have been demonstrated using h-BN and other 2D-nanomaterials. The synthesis of high-quality h-BN by chemical vapour deposition (CVD) has many challenges, some of which are similar to those of graphene where the right CVD parameters have to be found. But an inherent difficulty for the controlled h-BN production arises from the limited choice of suitable precursors containing both nitrogen and boron. A variety of precursors have been investigated, some of which are diborane and ammonia 8 , b-tricholoroborazine 9 , decaborane and ammonia 10 or borazine 11-18. For example, borazine allowed the growth of micron-sized h-BN domains on Cu at temperatures between 950-1000 °C 19 and is one of the most commonly used precursors due to its high vapour pressure. Nevertheless, these compounds are known to be toxic, highly flammable, corrosive, unstable, or difficult to store, presenting obvious safety concerns, practical challenges and higher processing costs. For this reason, ammonia borane (AB) precursor, classed as a non-hazardous substance

Customised transition metal oxide nanoparticles for the controlled production of carbon nanostructures

RSC Advances, 2012

Monodisperse magnetite (Fe3O4) and cobaltous oxide (CoO) nanoparticles were synthesised using a f... more Monodisperse magnetite (Fe3O4) and cobaltous oxide (CoO) nanoparticles were synthesised using a fast (up to 50 times quicker than previously reported) and facile one-pot, one-step reaction. The Fe3O4 nanoparticles had a constant size between 5 to 7 nm independent of the reagents used. Size controlled cube shaped CoO particles in the range of 10–20 nm were formed and it was possible to create voids in these particles in a controlled way. Carbon nanotubes were grown on these catalyst particles and their structures were compared. Structures of the nanotubes differed between straight, bamboo-like or partly coiled depending on the nanoparticle system used, suggesting the possibility of selection of these structures.

Direct visualization of electrical transport-induced alloy formation and composition changes in filled multi-wall carbon nanotubes by in situ scanning transmission electron microscopy

Journal of Alloys and Compounds, Oct 1, 2017

In order to evaluate the applicability of metal-filled carbon nanotubes, it is crucial to underst... more In order to evaluate the applicability of metal-filled carbon nanotubes, it is crucial to understand their behaviour in the presence of electric currents. In this work we exploit the significant advantages of combining in situ experiments with scanning transmission electron microscopy in annular dark-field mode and related analytical techniques to gain further insights into the electrical transport induced transformations in Fe-filled P,N-doped multi-wall carbon nanotubes. With these synergistic analysis techniques, we could monitor in real time the dynamical effects that take place in the carbon nanotubes and their Fe filling as a consequence of the passing of an electric current and subsequent Joule heating. A detailed analysis of the formation, evolution and composition of intermediate phases has been possible employing in situ experiments with transmission electron microscopy, scanning-transmission electron microscopy and analysis techniques. We show that a multistage process occurs in which the Fe filling reacts with nitrogen to form an intermediate alloy phase, which then decomposes. The presence of high concentrations of nitrogen within the inner channel of the tube is found to be crucial to this process.

Tuning the magnetic properties of iron-filled carbon nanotubes

Carbon, Aug 1, 2012

We tuned the magnetic properties of iron-filled multi-walled carbon nanotubes (Fe@CNTs) by carefu... more We tuned the magnetic properties of iron-filled multi-walled carbon nanotubes (Fe@CNTs) by careful manipulation of the synthesis parameters in a two zone furnace. Due to the small dimensions of iron filling in these samples they have some of the highest values of magnetization saturation reported for Fe@CNTs. Magnetic characterization of the Fe@CNTs revealed that their coercivity can be systematically tuned by varying the pyrolysis temperature, whereas the saturation magnetization can be significantly enhanced by optimization of the sublimation temperature. The rate of argon flow is seen to have little influence on either the saturation magnetization or the coercivity. Furthermore, the study of remnant magnetization as a function of time revealed that magnetization decay rates can be considerably varied.

Morphology – composition correlations in carbon nanotubes synthesised with nitrogen and phosphorus containing precursors

Physical Chemistry Chemical Physics, 2015

We have correlated the elemental composition with the structure of multi-wall carbon nanotubes sy... more We have correlated the elemental composition with the structure of multi-wall carbon nanotubes synthesised with nitrogen and phosphorus containing precursors and identified two chemically distinct dominant morphologies. The first type are cone-structured tubes and the second are nanotubes with fewer walls which can accommodate N 2 gas along their inner channel and contain up to ten times more nitrogen than the cone-structured nanotubes. Phosphorus was present in the catalyst particles but was not detected within the walls of either type of nanotube. Elemental analysis combined with in situ electrical measurements has allowed us to monitor the evolution of the doped nanotubes when current is passed. The N 2 gas becomes bonded immediately when current flows and the gas-containing nanotubes restructure more easily than the cone-structured ones. Since the inclusion of heteroatoms in multi-wall carbon nanotubes is generally inhomogeneous, understanding the distribution of elements across the sample is an important step towards the optimization of devices including gas sensors and components in electrical applications.

Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: II. An analytical study

Carbon, Jul 1, 2013

We report a study of aerosol-assisted chemical vapour deposition (ACVD) technique for the large s... more We report a study of aerosol-assisted chemical vapour deposition (ACVD) technique for the large scale production of carbon nanotubes (CNTs) by investigating the reasons behind the variation and inhomogeneity of the morphologies and quality of CNTs alongside the reactor. Our approach in understanding the system is to simplify it by dividing the reactor into sections and then to characterise the CNTs zone by zone using transmission and scanning electron microscopy, Raman spectroscopy and thermogravimetric analysis in order to identify their level of graphitisation, diameter distribution, catalyst content, growth rate and oxidation resistance in relation to their position in the reactor. Moreover, we established a gallery of electron micrographs depicting different morphologies of CNTs and identified the regions where multi-wall, double-wall, and single-wall nanotubes were produced. We conclude that any attempt to scale up the production of uniform CNTs by ACVD should first aim to homogenise the temperature and catalyst concentration throughout the reactor.

Electrical transport-induced transformations in filled carbon nanotubes imaged in situ by scanning-transmission electron microscopy

European Microscopy Congress 2016: Proceedings, Dec 20, 2016

Scanning Tunneling Microscopy and Spectroscopy of Nitrogen Doped Multi-Walled Carbon Nanotubes Produced by the Pyrolysis of Ferrocene and Benzylamine

Journal of Nanoscience and Nanotechnology, Oct 1, 2009

Scanning tunneling microscopy (STM) is used to visualize the structure of copper chloride and cob... more Scanning tunneling microscopy (STM) is used to visualize the structure of copper chloride and cobalt chloride graphite intercalation compounds (GICs). When the samples are biased negatively with respect to the tip, the images show details of the structure of the intercalant layers, and of its effects on the surface graphene layer. When the sample is under positive bias, symmetry properties of the uppermost graphite planes are revealed. Images of the CuCl 2 stage 1 GIC display a hexagonal symmetry in which all the atoms of the graphite surface plane appear. This is in contrast to the threefold symmetry usually seen in atomic resolution STM images of highly oriented pyrolytic graphite (HOPG), which we also observe on a reference sample of HOPG. The threefold symmetry is attributed to the ABAB stacking of the atomic layers in HOPG. In GICs, this stacking sequence is interrupted by the layer of intercalate, so that for the stage 1 compound all carbon atoms in the plane become equivalent, and six-fold symmetry develops. For a stage 2 (CuC1 2 or CoCl 2) GIC threefold symmetry is expected to persist. Images of the CoCl2 stage 1 GIC, taken with the sample bias is positive with respect to the tip, display a mixed trigonal and hexagonal symmetry, and may be attributable to the fact that the surface of sample is of mixed stage.

In situ engineering of NanoBud geometries

Chemical Communications, 2013

High resolution imaging was at 80kV was carried out using the Oxford JEOL JEM-2200MCO aberration-... more High resolution imaging was at 80kV was carried out using the Oxford JEOL JEM-2200MCO aberration-corrected, field-emission gun transmission electron microscope. A Gatan Ultrascan 4k x 4k CCD camera was used for image recording, at 1 second acquisition times. Aberration correction was undertaken analysing a phase plate (figure S1 a) resolved from a tableau of diffractograms taken a varying incident beam tilts (figure S1 b). Table S1 shows resulting values of beam aberrations prior to recording images.

N-SWCNTs production by aerosol-assisted CVD method

Chemical Physics Letters, Jun 1, 2012

The insertion of dopant atoms into the carbon nanotube framework allows one to tailor their elect... more The insertion of dopant atoms into the carbon nanotube framework allows one to tailor their electronic properties, reactivity and structure. We investigated the effect of the reaction parameters on the yield and quality of N-doped single-walled carbon nanotubes (N-SWCNTs) produced via aerosol chemical vapour deposition. In this context, aerosols of ethanol/benzylamine mixtures in conjunction with ferrocene were pyrolysed at temperatures between 950 and 1100°C. As a result, we were able to produce N-SWCNTs with a nitrogen content of ca. 1 at.%, diameters ranging between 0.9 and 1.8 nm, and production rates of more than 10 mg/h.

Controlled growth of Ni nanocrystals on SrTiO3 and their application in the catalytic synthesis of carbon nanotubes

Chemical Communications, 2013

Truncated pyramid-shaped Ni nanocrystals were epitaxially grown on SrTiO 3 (001) surfaces and cha... more Truncated pyramid-shaped Ni nanocrystals were epitaxially grown on SrTiO 3 (001) surfaces and characterised by scanning tunneling microscopy (STM). These nanocrystals were shown to be catalytically active for the synthesis of carbon nanotubes (CNTs). The narrow size distribution of the Ni nanocrystals results in a similar narrow distribution of CNT diameters.

Moderate strain induced indirect bandgap and conduction electrons in MoS2 single layers

npj 2D materials and applications, Oct 25, 2019

MoS 2 single layers are valued for their sizeable direct bandgap at the heart of the envisaged el... more MoS 2 single layers are valued for their sizeable direct bandgap at the heart of the envisaged electronic and optoelectronic applications. Here we experimentally demonstrate that moderate strain values (~2%) can already trigger an indirect bandgap transition and induce a finite charge carrier density in 2D MoS 2 layers. A conclusive proof of the direct-to-indirect bandgap transition is provided by directly comparing the electronic and optical bandgaps of strained MoS 2 single layers obtained from tunneling spectroscopy and photoluminescence measurements of MoS 2 nanobubbles. Upon 2% biaxial tensile strain, the electronic gap becomes significantly smaller (1.45 ± 0.15 eV) than the optical direct gap (1.73 ± 0.1 eV), clearly evidencing a strain-induced direct to indirect bandgap transition. Moreover, the Fermi level can shift inside the conduction band already in moderately strained (~2%) MoS 2 single layers conferring them a metallic character.

Tailoring MoS2 domains size, doping, and light emission by the sulfurization temperature of ultra-thin MoOx films on sapphire

Applied physics letters, Jun 10, 2024

Large-Area MoS2 Films Grown on Sapphire and GaN Substrates by Pulsed Laser Deposition

Nanomaterials, Oct 25, 2023

In this paper, we present the preparation of few-layer MoS 2 films on single-crystal sapphire, as... more In this paper, we present the preparation of few-layer MoS 2 films on single-crystal sapphire, as well as on heteroepitaxial GaN templates on sapphire substrates, using the pulsed laser deposition (PLD) technique. Detailed structural and chemical characterization of the films were performed using Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction measurements, and high-resolution transmission electron microscopy. According to X-ray diffraction studies, the films exhibit epitaxial growth, indicating a good in-plane alignment. Furthermore, the films demonstrate uniform thickness on large areas, as confirmed by Raman spectroscopy. The lateral electrical current transport of the MoS 2 grown on sapphire was investigated by temperature (T)-dependent sheet resistance and Hall effect measurements, showing a high n-type doping of the semiconducting films (n s from ~1 × 10 13 to ~3.4 × 10 13 cm -2 from T = 300 K to 500 K), with a donor ionization energy of E i = 93 ± 8 meV and a mobility decreasing with T. Finally, the vertical current injection across the MoS 2 /GaN heterojunction was investigated by means of conductive atomic force microscopy, showing the rectifying behavior of the I-V characteristics with a Schottky barrier height of φ B ≈ 0.36 eV. The obtained results pave the way for the scalable application of PLD-grown MoS 2 on GaN in electronics/optoelectronics.

Interface Properties of MoS2 van der Waals Heterojunctions with GaN

Nanomaterials, Jan 4, 2024

Waals Heterojunctions with GaN.

Atomic resolution interface structure and vertical current injection in highly uniform MoS2 heterojunctions with bulk GaN

Applied Surface Science, May 1, 2023

The integration of two-dimensional MoS2 with GaN recently attracted significant interest for futu... more The integration of two-dimensional MoS2 with GaN recently attracted significant interest for future electronic/optoelectronic applications. However, the reported studies have been mainly carried out using heteroepitaxial GaN templates on sapphire substrates, whereas the growth of MoS2 on lowdislocation-density bulk GaN can be strategic for the realization of "truly" vertical devices. In this paper, we report the growth of ultrathin MoS2 films, mostly composed by single-layers (1L), onto homoepitaxial n-GaN on n + bulk substrates by sulfurization of a pre-deposited MoOx film. Highly uniform and conformal coverage of the GaN surface was demonstrated by atomic force microscopy, while very low tensile strain (0.05%) and a significant p +-type doping (4.5×10 12 cm-2) of 1L-MoS2 was evaluated by Raman mapping. Atomic resolution structural and compositional analyses by aberration-corrected electron microscopy revealed a nearly-ideal van der Waals interface between MoS2 and the Ga-terminated GaN crystal, where only the topmost Ga atoms are affected by oxidation. Furthermore, the relevant lattice parameters of the MoS2/GaN heterojunction, such as the van der Waals gap, were measured with high precision. Finally, the vertical current injection across this 2D/3D heterojunction has been investigated by nanoscale current-voltage analyses performed by conductive atomic force microscopy, showing a rectifying behavior with an average turn-on voltage Von=1.7 V under forward bias, consistent with the expected band alignment at the interface between p + doped 1L-MoS2 and n-GaN.

Interface structure and doping of CVD grown MoS<sub>2</sub> on 4H‐SiC by microscopic analyses and <i>ab initio</i> calculations

Physica Status Solidi (rrl), Jul 25, 2023

The interface structure and electronic properties of monolayer (1L) MoS2 domains grown by chemica... more The interface structure and electronic properties of monolayer (1L) MoS2 domains grown by chemical vapor deposition on 4H–SiC(0001) are investigated by microscopic/spectroscopic analyses combined with ab initio calculations. The triangular domains are epitaxially oriented on the (0001) basal plane, with the presence of a van der Waals (vdW) gap between 1L–MoS2 and the SiC terraces. The high crystalline quality of the domains is confirmed by photoluminescence emission. Furthermore, a very low tensile strain (ε ≈ 0.03%) of 1L–MoS2, consistent with the small in‐plane lattice mismatch, and a p‐type doping of (0.45 ± 0.11) × 1013 cm−2, is evaluated by Raman mapping. Density functional theory (DFT) calculations of the MoS2/4H–SiC(0001) system are also performed, considering different levels of refinement of the model: 1) the simple case of the junction between Si‐terminated SiC and MoS2, showing a covalent bond between the Si–S atoms and n‐type doping of MoS2; 2) the complete passivation of Si dangling bonds with a monolayer (1 ML) of oxygen atoms, resulting in a vdW bond with dSi–S ≈ 3.84 Å bond length and p‐type doping of MoS2; and 3) partial (¼ ML and ½ ML) oxygen coverages of the 4H–SiC surface, resulting in intermediate values of dSi–S and doping behavior.

Indium Nitride at the 2D Limit

Advanced Materials, Nov 23, 2020

The properties of 2D InN are predicted to substantially differ from the bulk crystal. The predict... more The properties of 2D InN are predicted to substantially differ from the bulk crystal. The predicted appealing properties relate to strong in‐ and out‐of‐plane excitons, high electron mobility, efficient strain engineering of their electronic and optical properties, and strong application potential in gas sensing. Until now, the realization of 2D InN remained elusive. In this work, the formation of 2D InN and measurements of its bandgap are reported. Bilayer InN is formed between graphene and SiC by an intercalation process in metal–organic chemical vapor deposition (MOCVD). The thickness uniformity of the intercalated structure is investigated by conductive atomic force microscopy (C‐AFM) and the structural properties by atomic resolution transmission electron microscopy (TEM). The coverage of the SiC surface is very high, above 90%, and a major part of the intercalated structure is represented by two sub‐layers of indium (In) bonded to nitrogen (N). Scanning tunneling spectroscopy (STS) measurements give a bandgap value of 2 ± 0.1 eV for the 2D InN. The stabilization of 2D InN with a pragmatic wide bandgap and high lateral uniformity of intercalation is demonstrated.

Interface broadening due to Ar+ ion bombardment measured on Co/Cu multilayer at grazing angle of incidence

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2003

A Co/Cu multilayer system, with double layer thickness of about 60 nm was Auger electron spectros... more A Co/Cu multilayer system, with double layer thickness of about 60 nm was Auger electron spectroscopy depth profiled applying 1 keV Ar+ bombardment, varying the angle of incidence in the range of 78°–86°. It was found that (a) the relative sputter rate changes with the angle of incidence; (b) the interface broadening of the Co/Cu transition is larger than that of the Cu/Co transition and depends on the depth; (c) the broadening of the Co/Cu transition increases with increasing relative sputter rate; and (d) the broadening of the Cu/Co transition does not depend on the relative sputter rate. These features will be explained as the consequences of the strong preferential sputtering of Cu (with respect to Co) in this angular range. Because of the preferential sputtering ion bombardment induced roughening and smoothening (which was directly observed by atomic force microscopy measurements) occurs on the Co/Cu and Cu/Co interface, respectively. D-TRIM simulation has been also performed w...

Fókuszált ionsugaras és elektronsugaras nano-megmunkálás fizikai-kémiai alapjainak vizsgálata különös tekintettel a potenciális nanoelektronikai alkalmazásokra = Investigation of physical and chemical basics of focused electron and ion beam nano-machining with special emphasis on possible applica...

Részletes jelentés Az MTA Műszaki Fizikai és Anyagtudományi Kutatóintézet irányultsága a kétezres... more Részletes jelentés Az MTA Műszaki Fizikai és Anyagtudományi Kutatóintézet irányultsága a kétezres évek elejétől fokozottan fordult a nanoméretek fizikája és technológiája felé. Fontos lépés volt ezen az úton a pásztázó nanosugaras vizsgálati és megmunkálási módszerek "hadrendbe állítása"u.m.-kisenergiás pásztázó elektronmikroszkópia (LVSEM) és mikroanalízis,-mikroszkópia és nanoméretekben folytatott marás és mintázatok kialakítása fókuszált ionsugár (FIB) segítségével az LVSM és a FIB sugarát felhasználva gázfázisú prekurzorok injektálásával (GIS) anyagleválasztás nanoméretekben A LVSEM lehetőségei: milliószoros nagyítás szigetelő Al2O3 szemcséről, és különféle C nanocsövek képei brought to you by CORE View metadata, citation and similar papers at core.ac.uk

Time dependent decomposition of ammonia borane for the controlled production of 2D hexagonal boron nitride

Scientific Reports, Oct 30, 2017

Ammonia borane (AB) is among the most promising precursors for the large-scale synthesis of hexag... more Ammonia borane (AB) is among the most promising precursors for the large-scale synthesis of hexagonal boron nitride (h-BN) by chemical vapour deposition (CVD). Its non-toxic and non-flammable properties make AB particularly attractive for industry. AB decomposition under CVD conditions, however, is complex and hence has hindered tailored h-BN production and its exploitation. To overcome this challenge, we report in-depth decomposition studies of AB under industrially safe growth conditions. In situ mass spectrometry revealed a time and temperature-dependent release of a plethora of N x B y-containing species and, as a result, significant changes of the N:B ratio during h-BN synthesis. Such fluctuations strongly influence the formation and morphology of 2D h-BN. By means of in situ gas monitoring and regulating the precursor temperature over time we achieve uniform release of volatile chemical species over many hours for the first time, paving the way towards the controlled, industrially viable production of h-BN. Hexagonal boron nitride (h-BN) is a layered ceramic material with an atomic structure similar to that of graphite, but with a wide band gap of 5.7 eV 1. Monolayer or few-layer h-BN has received considerable attention recently as a substrate for graphene and other 2D-nanomaterials. Due to the low number of charge impurities in h-BN the mobility of supported graphene improved by more than an order of magnitude 2 compared to graphene on the commonly used silicon dioxide. Highly crystalline h-BN domains serve as a better template for the epitaxial growth of monolayer graphene 3 and WS 2 4 , showing good alignment of the domains. The possibility of forming heterostructures wholly from the 2D family of nanomaterials opens many exciting opportunities in the electronics industry. Radio frequency electronics 5 , nanocapacitors 6 , flexible and transparent transistors 7 have been demonstrated using h-BN and other 2D-nanomaterials. The synthesis of high-quality h-BN by chemical vapour deposition (CVD) has many challenges, some of which are similar to those of graphene where the right CVD parameters have to be found. But an inherent difficulty for the controlled h-BN production arises from the limited choice of suitable precursors containing both nitrogen and boron. A variety of precursors have been investigated, some of which are diborane and ammonia 8 , b-tricholoroborazine 9 , decaborane and ammonia 10 or borazine 11-18. For example, borazine allowed the growth of micron-sized h-BN domains on Cu at temperatures between 950-1000 °C 19 and is one of the most commonly used precursors due to its high vapour pressure. Nevertheless, these compounds are known to be toxic, highly flammable, corrosive, unstable, or difficult to store, presenting obvious safety concerns, practical challenges and higher processing costs. For this reason, ammonia borane (AB) precursor, classed as a non-hazardous substance

Customised transition metal oxide nanoparticles for the controlled production of carbon nanostructures

RSC Advances, 2012

Monodisperse magnetite (Fe3O4) and cobaltous oxide (CoO) nanoparticles were synthesised using a f... more Monodisperse magnetite (Fe3O4) and cobaltous oxide (CoO) nanoparticles were synthesised using a fast (up to 50 times quicker than previously reported) and facile one-pot, one-step reaction. The Fe3O4 nanoparticles had a constant size between 5 to 7 nm independent of the reagents used. Size controlled cube shaped CoO particles in the range of 10–20 nm were formed and it was possible to create voids in these particles in a controlled way. Carbon nanotubes were grown on these catalyst particles and their structures were compared. Structures of the nanotubes differed between straight, bamboo-like or partly coiled depending on the nanoparticle system used, suggesting the possibility of selection of these structures.

Direct visualization of electrical transport-induced alloy formation and composition changes in filled multi-wall carbon nanotubes by in situ scanning transmission electron microscopy

Journal of Alloys and Compounds, Oct 1, 2017

In order to evaluate the applicability of metal-filled carbon nanotubes, it is crucial to underst... more In order to evaluate the applicability of metal-filled carbon nanotubes, it is crucial to understand their behaviour in the presence of electric currents. In this work we exploit the significant advantages of combining in situ experiments with scanning transmission electron microscopy in annular dark-field mode and related analytical techniques to gain further insights into the electrical transport induced transformations in Fe-filled P,N-doped multi-wall carbon nanotubes. With these synergistic analysis techniques, we could monitor in real time the dynamical effects that take place in the carbon nanotubes and their Fe filling as a consequence of the passing of an electric current and subsequent Joule heating. A detailed analysis of the formation, evolution and composition of intermediate phases has been possible employing in situ experiments with transmission electron microscopy, scanning-transmission electron microscopy and analysis techniques. We show that a multistage process occurs in which the Fe filling reacts with nitrogen to form an intermediate alloy phase, which then decomposes. The presence of high concentrations of nitrogen within the inner channel of the tube is found to be crucial to this process.

Tuning the magnetic properties of iron-filled carbon nanotubes

Carbon, Aug 1, 2012

We tuned the magnetic properties of iron-filled multi-walled carbon nanotubes (Fe@CNTs) by carefu... more We tuned the magnetic properties of iron-filled multi-walled carbon nanotubes (Fe@CNTs) by careful manipulation of the synthesis parameters in a two zone furnace. Due to the small dimensions of iron filling in these samples they have some of the highest values of magnetization saturation reported for Fe@CNTs. Magnetic characterization of the Fe@CNTs revealed that their coercivity can be systematically tuned by varying the pyrolysis temperature, whereas the saturation magnetization can be significantly enhanced by optimization of the sublimation temperature. The rate of argon flow is seen to have little influence on either the saturation magnetization or the coercivity. Furthermore, the study of remnant magnetization as a function of time revealed that magnetization decay rates can be considerably varied.

Morphology – composition correlations in carbon nanotubes synthesised with nitrogen and phosphorus containing precursors

Physical Chemistry Chemical Physics, 2015

We have correlated the elemental composition with the structure of multi-wall carbon nanotubes sy... more We have correlated the elemental composition with the structure of multi-wall carbon nanotubes synthesised with nitrogen and phosphorus containing precursors and identified two chemically distinct dominant morphologies. The first type are cone-structured tubes and the second are nanotubes with fewer walls which can accommodate N 2 gas along their inner channel and contain up to ten times more nitrogen than the cone-structured nanotubes. Phosphorus was present in the catalyst particles but was not detected within the walls of either type of nanotube. Elemental analysis combined with in situ electrical measurements has allowed us to monitor the evolution of the doped nanotubes when current is passed. The N 2 gas becomes bonded immediately when current flows and the gas-containing nanotubes restructure more easily than the cone-structured ones. Since the inclusion of heteroatoms in multi-wall carbon nanotubes is generally inhomogeneous, understanding the distribution of elements across the sample is an important step towards the optimization of devices including gas sensors and components in electrical applications.

Aerosol-assisted chemical vapour deposition synthesis of multi-wall carbon nanotubes: II. An analytical study

Carbon, Jul 1, 2013

We report a study of aerosol-assisted chemical vapour deposition (ACVD) technique for the large s... more We report a study of aerosol-assisted chemical vapour deposition (ACVD) technique for the large scale production of carbon nanotubes (CNTs) by investigating the reasons behind the variation and inhomogeneity of the morphologies and quality of CNTs alongside the reactor. Our approach in understanding the system is to simplify it by dividing the reactor into sections and then to characterise the CNTs zone by zone using transmission and scanning electron microscopy, Raman spectroscopy and thermogravimetric analysis in order to identify their level of graphitisation, diameter distribution, catalyst content, growth rate and oxidation resistance in relation to their position in the reactor. Moreover, we established a gallery of electron micrographs depicting different morphologies of CNTs and identified the regions where multi-wall, double-wall, and single-wall nanotubes were produced. We conclude that any attempt to scale up the production of uniform CNTs by ACVD should first aim to homogenise the temperature and catalyst concentration throughout the reactor.

Electrical transport-induced transformations in filled carbon nanotubes imaged in situ by scanning-transmission electron microscopy

European Microscopy Congress 2016: Proceedings, Dec 20, 2016

Scanning Tunneling Microscopy and Spectroscopy of Nitrogen Doped Multi-Walled Carbon Nanotubes Produced by the Pyrolysis of Ferrocene and Benzylamine

Journal of Nanoscience and Nanotechnology, Oct 1, 2009

Scanning tunneling microscopy (STM) is used to visualize the structure of copper chloride and cob... more Scanning tunneling microscopy (STM) is used to visualize the structure of copper chloride and cobalt chloride graphite intercalation compounds (GICs). When the samples are biased negatively with respect to the tip, the images show details of the structure of the intercalant layers, and of its effects on the surface graphene layer. When the sample is under positive bias, symmetry properties of the uppermost graphite planes are revealed. Images of the CuCl 2 stage 1 GIC display a hexagonal symmetry in which all the atoms of the graphite surface plane appear. This is in contrast to the threefold symmetry usually seen in atomic resolution STM images of highly oriented pyrolytic graphite (HOPG), which we also observe on a reference sample of HOPG. The threefold symmetry is attributed to the ABAB stacking of the atomic layers in HOPG. In GICs, this stacking sequence is interrupted by the layer of intercalate, so that for the stage 1 compound all carbon atoms in the plane become equivalent, and six-fold symmetry develops. For a stage 2 (CuC1 2 or CoCl 2) GIC threefold symmetry is expected to persist. Images of the CoCl2 stage 1 GIC, taken with the sample bias is positive with respect to the tip, display a mixed trigonal and hexagonal symmetry, and may be attributable to the fact that the surface of sample is of mixed stage.

In situ engineering of NanoBud geometries

Chemical Communications, 2013

High resolution imaging was at 80kV was carried out using the Oxford JEOL JEM-2200MCO aberration-... more High resolution imaging was at 80kV was carried out using the Oxford JEOL JEM-2200MCO aberration-corrected, field-emission gun transmission electron microscope. A Gatan Ultrascan 4k x 4k CCD camera was used for image recording, at 1 second acquisition times. Aberration correction was undertaken analysing a phase plate (figure S1 a) resolved from a tableau of diffractograms taken a varying incident beam tilts (figure S1 b). Table S1 shows resulting values of beam aberrations prior to recording images.

N-SWCNTs production by aerosol-assisted CVD method

Chemical Physics Letters, Jun 1, 2012

The insertion of dopant atoms into the carbon nanotube framework allows one to tailor their elect... more The insertion of dopant atoms into the carbon nanotube framework allows one to tailor their electronic properties, reactivity and structure. We investigated the effect of the reaction parameters on the yield and quality of N-doped single-walled carbon nanotubes (N-SWCNTs) produced via aerosol chemical vapour deposition. In this context, aerosols of ethanol/benzylamine mixtures in conjunction with ferrocene were pyrolysed at temperatures between 950 and 1100°C. As a result, we were able to produce N-SWCNTs with a nitrogen content of ca. 1 at.%, diameters ranging between 0.9 and 1.8 nm, and production rates of more than 10 mg/h.

Controlled growth of Ni nanocrystals on SrTiO3 and their application in the catalytic synthesis of carbon nanotubes

Chemical Communications, 2013

Truncated pyramid-shaped Ni nanocrystals were epitaxially grown on SrTiO 3 (001) surfaces and cha... more Truncated pyramid-shaped Ni nanocrystals were epitaxially grown on SrTiO 3 (001) surfaces and characterised by scanning tunneling microscopy (STM). These nanocrystals were shown to be catalytically active for the synthesis of carbon nanotubes (CNTs). The narrow size distribution of the Ni nanocrystals results in a similar narrow distribution of CNT diameters.

Moderate strain induced indirect bandgap and conduction electrons in MoS2 single layers

npj 2D materials and applications, Oct 25, 2019

MoS 2 single layers are valued for their sizeable direct bandgap at the heart of the envisaged el... more MoS 2 single layers are valued for their sizeable direct bandgap at the heart of the envisaged electronic and optoelectronic applications. Here we experimentally demonstrate that moderate strain values (~2%) can already trigger an indirect bandgap transition and induce a finite charge carrier density in 2D MoS 2 layers. A conclusive proof of the direct-to-indirect bandgap transition is provided by directly comparing the electronic and optical bandgaps of strained MoS 2 single layers obtained from tunneling spectroscopy and photoluminescence measurements of MoS 2 nanobubbles. Upon 2% biaxial tensile strain, the electronic gap becomes significantly smaller (1.45 ± 0.15 eV) than the optical direct gap (1.73 ± 0.1 eV), clearly evidencing a strain-induced direct to indirect bandgap transition. Moreover, the Fermi level can shift inside the conduction band already in moderately strained (~2%) MoS 2 single layers conferring them a metallic character.