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Publications (109)
The formation of germanene on Ag(111) has been extensively studied and confirmed. However, the nature of the transition stage between the initial A g 2 Ge alloy phase at 1/3 monolayer (ML) and the final quasifreestanding phase germanene phase at coverage ≥ 1 ML still remains elusive. Using angle-resolved photoemission spectroscopy, scanning-tunneli...
Alkali atom doping is an efficient way to induce charge transfer and Fermi level tuning in layered materials through intercalation. However, there is a general lack of microscopic understanding of the effect of doping inhomogeneity in geometric and electronic aspects. Here, we report surface doping of a bulk VSe2 crystal by sodium. Na atoms form in...
We identify and manipulate commonly occurring defects in black phosphorus, combining scanning tunneling microscopy experiments with density functional theory calculations. A ubiquitous defect, imaged at negative bias as a bright dumbbell extending over several nanometers, is shown to arise from a substitutional Sn impurity in the second sublayer. A...
Hetero-structures of transition metal dichalcogenide (TMD) layers offer vast possibilities of new materials properties. The growth behaviors of such TMD hetero-structures can be complex. In this study, we report the structure modification of a VSe 2 monolayer by the molecular beam deposition of Te, V, or both. With a typical growth temperature of 3...
Growth of Xene, referred to as the analog of graphene made of atoms other than carbon, with higher lattice quality and more intrinsic electronic structures, is an important challenge to the development of two-dimensional (2D) materials. We report an approach using Pb-atom deposition to striped-phase (SP) germanene grown on Ag(111) to trigger it to...
We report on high temperature (T) studies of conductivity, σ, of the monoclinic phase of NbS3 (NbS3-II). The compound is shown to be stable up to T∼550 K. Threshold non-linear conductivity and Shapiro steps on the I – V curves can be seen up to TP0 ≈ 450 – 475 K clearly demonstrating sliding of a charge-density wave (CDW). The feature in σ(T) shows...
We demonstrate that the linewidth of the field emission resonance (FER) observed on the surface of MoS2 using scanning tunneling microscopy can vary by up to one order of magnitude with an increasing electric field. This phenomenon originates from quantum trapping, in which the electron relaxed from a resonant electron in the FER is momentarily tra...
Field-effect transistor structures based on whiskers of layered quasi-one-dimensional semiconductor TiS3 have been fabricated. The dependences of the conductivity σ on the gate voltage Vg, as well as the current-voltage characteristics of whiskers (“source-drain”) at different Vg values, have been measured in the temperature range of 4.2-300 K. As...
We report studies of the Seebeck coefficient S of the quasi-one-dimensional compound NbS3, together with the temperature dependence of its specific conductivity σs and heat capacity cp. The monoclinic phase (NbS3-II) is studied over the temperature range T=80–400 K, which covers two charge density wave (CDW) transitions at TP1=360 K (CDW-1) and TP2...
We report the evolution of charge density wave states under pressure for two NbS3 phases: triclinic (phase I) and monoclinic (phase II) at room temperature. Raman and x-ray diffraction (XRD) techniques are applied. The x-ray studies on the monoclinic phase under pressure show a compression of the lattice at different rates below and above ∼7 GPa bu...
Ultrathin Pb and Ge films deposited on Ag(111) surfaces have been investigated and compared. We found that at 1/3 ML, both films formed surface alloys, Ag 2 Pb and Ag 2 Ge, with √ 3 × √ 3R30 • and 19 20 √ 3 × 19 20 √ 3R30 • structures on Ag(111) but the surface electronic structures exhibit a most evident difference at the Ag(111) surface zone boun...
We demonstrate that the linewidth of the field emission resonance (FER) observed on the surface of MoS2 using scanning tunneling microscopy can vary by up to one order of magnitude with an increase in the electric field. This unusual phenomenon originates from quantum trapping, where the electron relaxed from the resonant electron in the FER is mom...
DOI:https://doi.org/10.1103/PhysRevB.99.039901
Single layers of transition metal dichalcogenides (TMDCs) are excellent candidates for electronic applications beyond the graphene platform; many of them exhibit novel properties including charge density waves (CDWs) and magnetic ordering. CDWs in these single layers are generally a planar projection of the corresponding bulk CDWs because of the qu...
Single layers of transition metal dichalcogenides (TMDCs) are excellent candidates for electronic applications beyond the graphene platform; many of them exhibit novel properties including charge density waves (CDWs) and magnetic ordering. CDWs in these single layers are generally a planar projection of the corresponding bulk CDWs because of the qu...
Two-dimensional (2D) topological insulators (TIs) are promising platforms for low-dissipation spintronic devices based on the quantum spin Hall (QSH) effect, but experimental realization of such systems with a large band gap suitable for room-temperature applications has proven difficult. Here, we report the successful growth on bilayer graphene of...
Two-dimensional (2D) topological insulators (TIs) are promising platforms for low-dissipation spintronic devices based on the quantum spin Hall (QSH) effect, but experimental realization of such systems with a large band gap suitable for room-temperature applications has proven difficult. Here, we report the successful growth on bilayer graphene of...
Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe$_2$ from the single-layer to the bulk lim...
The basic and the charge density wave (CDW) structures of the monoclinic $NbS_3-II$ polymorph were studied by synchrotron x-ray diffraction, ab-initio calculation, simulation of electron diffraction patterns and by atomic-resolution transmission electron and low-temperature scanning tunneling microscopies. It is shown that the basic structure belon...
The basic and the charge density wave (CDW) structures of the monoclinic $NbS_3-II$ polymorph were studied by synchrotron x-ray diffraction, ab-initio calculation, simulation of electron diffraction patterns and by atomic-resolution transmission electron and low-temperature scanning tunneling microscopies. It is shown that the basic structure belon...
The form of the external potential (FEP) for generating field emission resonance (FER) in a scanning tunneling microscopy (STM) junction is usually assumed to be triangular. We demonstrate that this assumption can be examined using a plot that can characterize FEP. The plot is FER energies versus the corresponding distances between the tip and samp...
Two-dimensional (2D) honeycomb lattices beyond graphene promise new physical properties such as quantum spin Hall effect. While there have been claims of growth of such lattices (silicene, germanene, stanene), their existence needs further support and their preparation and characterization remain a difficult challenge. Our findings suggest that two...
Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe2 from the single-layer to the bulk limit....
Direct growth of graphene integrated into electronic devices is highly desirable but difficult due to the nominal ~1000 °C chemical vapor deposition (CVD) temperature, which can seriously deteriorate the substrates. Here we report a great reduction of graphene CVD temperature, down to 50 °C on sapphire and 100 °C on polycarbonate, by using dilute m...
DOI:https://doi.org/10.1103/PhysRevB.96.119903
A strictly two-dimensional (2D) material such as freestanding graphene (FSG) is rarely investigated at the atomic scale by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). A basic difficulty in probing FSG by STM and STS is the mechanical instability when a highly compliant 2D atomic layer interacts with a proximal tip...
A systematic study of polycrystalline 1T−TiSe2−δ with controlled Se loss indicates that the unconventional charge density wave (CDW) phase is found to be most pronounced in samples with δ∼0.12, instead of being Se vacancy free. The level of Se vacancy defects and temperature determines whether 1T−TiSe2−δ should be categorized as a semiconductor, a...
After the discovery of graphene and its consequences in the field of nanoscience and nanomaterials, there has been a growing interest in 2D materials and also their vertical stacking due to unique properties and potential applications.[1] For instance, it was shown the transport properties of exfoliated graphene supported by hexagonal boron nitride...
Book of extended abstracts of papers presented at the international conference ACSIN 2016
held in Frascati, Rome, Italy October 9-15, 2016 Organized by the Non profit organization for scientific research Superstripes onlus, Rome International Center for Materials Science Superstripes - RICMASS. Conference chairs
Antonio Bianconi - RICMASS Rome, Ita...
Through transport, compositional and structural studies, we review the features of the charge-density wave (CDW) conductor of NbS$_{3}$ (phase II). We highlight three central results: 1) In addition to the previously reported CDW transitions at $T_{P1}$ = 360\,K and $T_{P2}$ = 150\,K, another CDW transition occurs at a much higher temperature $T_{P...
Through transport, compositional and structural studies, we review the features of the charge-density wave (CDW) conductor of NbS$_{3}$ (phase II). We highlight three central results: 1) In addition to the previously reported CDW transitions at $T_{P1}$ = 360\,K and $T_{P2}$ = 150\,K, another CDW transition occurs at a much higher temperature $T_{P...
We demonstrate that a C 60 overlayer enhances the perpendicular magnetic anisotropy of a Co thin film, inducing an inverse spin reorientation transition from in plane to out of plane. The driving force is the C 60 =Co interfacial magnetic anisotropy that we have measured quantitatively in situ as a function of the C 60 coverage. Comparison with sta...
The {\pi}-plasmon dispersion in graphene was scrutinized by
momentum(q)-resolved electron energy-loss spectroscopy with an improved q
resolution and found to display the square root of q dispersion characteristic
of the collective excitation of two-dimensional electron systems, in contrast
with previous experimental and theoretical studies which re...
We present a scanning tunneling microscopy study of the stress–strain behavior of a rippled single-layer free-standing graphene (FSG) and report that FSG exhibits a non-linear sigmoidal stress–strain behavior. We managed to pull and push nanoripples by controlling the STM tip–FSG interaction forces. In this way, we found that the initial deformatio...
We present an X-ray photoelectron spectroscopy depth profiling study of the size and volumetric distributions of Pt nanoparticles supported on two mesoporous carbons. Based on the analysis of the chemical shift of the Pt4f peaks, we found the one with a mesopore diameter of ∼5 nm has a uniform distribution of crystalline Pt nanoparticles throughout...
We present the density functional calculations for the adsorption of a single platinum (Pt) atom on the 2-fold bridged-oxygen (Ob) covered graphene (graphene oxide, GO). We found the incoming Pt at low kinetic energies prefers to interact with only one Ob and the reaction weakens the coupling of the Ob with the underlying two carbon atoms to give r...
Step fluctuations on Ag(110) surfaces have been investigated with STMT atomic events that underlie these thermal fluctuations are quantified using a Langevin analysis. From the t1- scaling of the step-position correlation function, we deduce random attachment and detachment processes along the step edge, with a characteristic interval of 350 ms bet...
Multilayer Ge/Si quantum wells grown on Si(100) at lower temperatures (300 C for Ge and 435 C for Si) exhibit unexpected large surface undulations. These groove-like features along 〈110〉 directions are correlated to the underlying dislocation networks. Studies on the dependence of surface roughness versus Ge spacer thickness exclude kinetic limitat...
We have investigated the growth and structure of a C60 monolayer film on Cu(001) with scanning tunneling microscopy at room temperature and 100 K. We discovered that the equilibrium adsorption structure of annealed C60 films depends sensitively on the initial deposition coverage; for a coverage of 0.5 monolayer C60 orders in an one-bright-and-one-d...
We demonstrate the charge state of C60 on a Cu(111) surface can be made optimal, i.e., forming C60(3-) as required for superconductivity in bulk alkali-doped C60, purely through interface reconstruction rather than with foreign dopants. We link the origin of the C60(3-) charge state to a reconstructed interface with ordered (4x4) 7-atom vacancy hol...
We theoretically predict the occurrence of negative differential resistance (NDR) in scanning tunneling spectroscopy for a pure C60 monolayer deposited on a metal surface using metal tips, namely, on a Cu(111) surface and using various W tips. It is proposed that the likely reason why NDR has not been observed under such conditions is that NDR can...
The clean-off reaction of AgO added rows by CO on Ag(110) and Au/Ag(110) bimetallic surfaces was studied by scanning tunneling microscopy (STM) and compared with density functional theory (DFT). This combined study of a model system illustrated the complexity of catalytic enhancement in bimetallic systems. By analyzing in situ time-lapsed STM image...
Fluorescent nanodiamond (FND) contains nitrogen-vacancy defect centers as fluorophores. The intensity of its fluorescence can be significantly enhanced after deposition of the particle (35 or 140 nm in size) on a nanocrystalline Ag film without a buffer layer. The excellent photostability (i.e. neither photobleaching nor photoblinking) of the mater...
We report an unusual phenomena of attraction between DNA and the sidewall of a two-dimensional (2d) glass nanoslit with height, h, less than the Kuhn length l(K). The DNA molecules are stretched and diffuse along the wall. The scaling analysis reveals that the wall-bound DNA molecules exhibit one-dimensional (1d) diffusion. A modified blob model is...
The correlation between morphology and charge-carrier mobility in the vertical direction in thin films of poly(2-methoxy-5-(2'- ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) is investigated by a combination of X-ray reflectivity (XRR), field-emis- sion scanning electron microscopy (FESEM), atomic force microscopy (AFM), fluorescence optical micro...
In the rich phase diagram of NaxCoO2, x=0.71 enjoys special stability and is called the Curie-Weiss metal due to its anomalous properties. Similarly, x=0.84 prepared from high temperature melt is a special end point beyond which the system phase separates. Using synchrotron x-ray diffraction on single crystals, we discovered sqrt[12]a and sqrt[13]a...
Sodium ion ordering on an in situ cleaved NaxCoO2 (x=0.84) surface has been studied by ultrahigh vacuum scanning tunneling microscopy at room temperature. Three main phases, with p(3 x 3), ( radical7 x radical7), and (2 radical3 x 2 radical3) hexagonal unit cells and a surface Na concentration of 1/3, 3/7, 1/2, respectively, were identified. One su...
Sodium ion ordering on in situ cleaved NaxCoO2 (x=0.84) surface has been studied by ultra high vacuum scanning tunneling microscopy (UHV-STM) at room temperature. Three main phases, with p(3x3), (root 7 x root 7), and (2 root 3 x 2 root 3) hexagonal unit cells and surface Na concentration of 1/3, 3/7, 1/2, respectively, were identified. One surpris...
We have recently extended to nanostructures the basic theoretical capabilities of surface structure determination by Low Energy Electron Diffraction (LEED), by adopting a non-periodic cluster approach and substantially accelerating the computation time for complex structures. In this contribution, we describe two further theoretical enhancements an...
The migration behavior of C60 on Ni(111) has been inferred from its growth morphology at various substrate temperatures, as observed with scanning tunneling microscopy. The number density of islands increased and their average sizes decreased anomalously in the temperature range of approximately 573 K to approximately 973 K. This trend contradicts...
The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hamper...
We report a scanning tunneling microscopy (STM) study of potassium (K)-doped C60 (KxC60) on Cu(111) and Ag(111), with focus on the observation of novel K-induced superstructures. On Cu(111), K-intercalation leads to various superstructures, i.e., p(3 × 3), p(2 × 2), and a disordered "maze-like" structure. Concomitant removal of C60-induced substrat...
Using scanning tunneling microscopy and spectroscopy and first principles calculations, we have systematically studied the morphological and electronic structures of a C60 monolayer on Ag(100). Our results reveal that the bright-dim contrast has a definite geometric origin and there are two types of dim C60 molecules, one a monomer and the other a...
Rutile iridium dioxide (IrO2) surfaces were studied by cross-sectional scanning tunneling microscopy (XSTM). Atomically flat surfaces prepared by in situ ultra high vacuum cleaving of crystalline platelets of thicknesses < 50 mu m were successfully demonstrated. In addition to (110) surface, several vicinal planes, e.g., (120), (130) and similar to...
The authors have used cross-sectional scanning tunneling microscopy to examine strain relaxation profiles of InAs quantum dots with In0.33Ga0.67As layers overgrown by three distinct capping methods. A statistical analysis of strain relaxation profile allowed them to infer that the long wavelength emission (>1.3 μm) of InAs quantum dots capped with...
The conducting layered sodium cobaltate, NaxCoO2, has generated great research interests recently. This material exhibits surprising properties as the Na concentration x is varied. Despite intense studies, Na ordering and its subtle interplay with charge ordering in the CoO2 layer remains unclear. Here we report the first direct observation of Na o...
Fullerene C-60 and C-70 Mixture films oil Ag(100) have been Studied with scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). Ill contrast to the recently found aperiodic incommensurate "(111)" phase of C-60 oil Ag( 100), C-70 formed a commensurate c(6 x 4) Structure. Co-deposited C-60/C-70 mixture layers were examined as...
Formation of two-dimensional islands of methyl radical (CH3) and Cu(I 11) and CU(I 10) Surfaces, and methylnitrene radical (NCH3) islands on Cu(I 10) was studied with scanning tunneling microscopy. In all cases, close-packed or nearly close-packed two-dimensional islands were observed. We found that (1) chemisorbed CH3 aggregated into a (root 3- x...
C60 adsorption on metal surfaces typically incurs different amount of charge transfer from the substrate to the molecules. The charge transfer amount has never been found to approach that of optimally doped fulleride, e.g., K3C60, in which three electrons occupy the C60 LUMO states. Here we demonstrate that C60 adsorbed on Cu(111) render a nearly o...
The adsorption structure of C60 on Ag(100) is a very interesting one because it is a uniaxial incommensurate aperiodic (111) phase with extensive adsorbate-induced reconstruction [1]. Peculiar molecular ordering arises therein due to delicate balance of competitive interactions. In contrast, few adsorption studies were conducted for the next abunda...
Understanding molecular ordering is a critical step in achieving molecular self-assembly for the fabrication of nanomaterials, and molecular ordering in the adsorption of large molecules on atomically flat surfaces can be characterized with precision by scanning tunneling microscopy (STM). Complications arise therein from the expanded possibility o...
We report direct STM observation of the formation of two-dimensional islands of methyl radical (CH3) on Cu(111) and Cu(110) surfaces, and methylnitrene radical (NCH3) islands on Cu(110). In corroboration with LEED measurements, plausible adsorption models of each system are proposed. In all studied cases, first-order reaction kinetics of long-chain...
Using a simplified yet realistic strain-relaxation model, we analyse facet-resolved non-contact AFM (NC-AFM) data of self-assembled strained germanium islands grown on Si(100) at 650 °C and extract estimates of the nucleation barrier and pyramid-to-dome shape transition barrier. We estimate the shape transition barrier of Ge dots as , and the nucle...
We revisit submonolayer growth of C60 on Cu(111) by scanning tunneling microscopy (STM), with emphasis on the formation of higher-order commensurate metastable states. These phases show concomitant interfacial reconstruction, adlayer buckling, and adlayer rotation in order to match as closely as possible the 10.0 Å C60 nearest neighbor (NN) distanc...
The formation of methyl radicals into densely packed two-dimensional islands on the Cu(111) surface is directly observed for the first time by scanning tunneling microscopy (STM). It is shown that chemisorbed CH3(ads) molecules tend to aggregate into ( × ) R30° structure in 2D islands at any coverage up to one saturation monolayer. The island sizes...
With combined VT-STM and LEED measurements, the debated nature of distinct bright-dim C60 contrast and its complicated ordering on Ag(100) is clarified. Emergence of the bright-dim contrast above 280 K is shown to be an irreversible and thermally activated process. Observation of substrate mass flow verifies that this contrast arises from an adsorb...
Detailed combined scanning tunneling microscopy and low-energy electron diffraction measurements reveal that the structure of a C60 monolayer on Ag(100) is not the previously accepted commensurate c(6 × 4) phase but rather an incommensurate (111) close-packed phase. The film exhibits a characteristic molecular contrast pattern with merely short-ran...
The nature of distinct bright-dim ${\mathrm{C}}_{60}$ contrast and its intricate ordering on Ag(100) is clarified with scanning tunneling microscopy. The ${\mathrm{C}}_{60}$ film is (111) close-packed with locally distorted rhombic units of specific contrast order, forming an incommensurate aperiodic phase. We relate the ${\mathrm{C}}_{60}$ contras...
Monolayer C_60 grown on Ag(100) reveals intriguing electronic phase transition and a strong ¡§bright¡¨ and ¡§dim¡¨ molecular contrast with merely short range order in STM. All these findings are not well understood. With a focus variable temperature STM study, the debated nature of the distinct bright-dim molecular contrast and its complica...
In the c(6×4) C60 layer on Ag(1 0 0), a strong “bright” and “dim” molecular contrast has recently been observed in scanning tunneling microscopy. While the nature of such contrast is still under debate, substrate reconstruction is considered unlikely due to the rather complicated local ordering in the C60 layer. Here we use variable temperature STM...
Both electrical and optical reliabilities of PMOS and NMOS tunneling diodes are enhanced by oxide roughness, prepared by very high vacuum prebake technology. For rough PMOS devices, as compared to flat PMOS devices, the Weibull plot of T/sub BD/ shows a 2.5-fold enhancement at 63% failure rate, while both the D/sub 2/ and H/sub 2/-treated flat PMOS...
We present a combined study by scanning tunneling microscopy and atomistic simulations of the emission of dissociated dislocation loops by nanoindentation on a (001) fcc surface. The latter consist of two stacking-fault ribbons bounded by Shockley partials and a stair-rod dislocation. These dissociated loops, which intersect the surface, are shown...
A pinch-off phenomenon is discovered in the evolution of 2D wormlike nanoclusters formed in homoepitaxial adlayers. This feature is shown to distinguish mass transport via periphery diffusion from other mechanisms. Continuum modeling of such evolution accurately describes experimental observations, particularly if one incorporates the anisotropy in...
Systems and methods are described for fabricating arrays of quantum dots. A method for making a quantum dot device, includes: forming clusters of atoms on a substrate; and charging the clusters of atoms such that the clusters of atoms repel one another. The systems and methods provide advantages because the quantum dots can be ordered with regard t...
During submonolayer homoepitaxy, by controlling the amount of material deposited, one can form clusters of both adatoms and vacancies with shapes far from their equilibrium forms. Subsequent evolution of those clusters is governed by both step edge energetics and transport mechanisms. Time-sequenced STM measurements are performed for such clusters...
We have studied the molecular orientation of ferrocene, Fe(eta^5-C_5H_5)_2, on Ag(100), Cu(100) and Mo(112) using angle resolved photoemission, high resolution electron energy loss and variable temperature scanning tunneling microscopy. There is strong evidence that the molecular bonding orientation is dependent upon substrate. On Cu(100) and Mo(11...
We present a novel approach for room-temperature molecular manipulation which exploits the reduced thermal mobility of chemisorbed molecular radicals, with cyclopentadienyl, C5H5, on Ag(100) as an example. Several novel manipulation modes are demonstrated, all achieved at room temperature. Most notably, the dynamics of the radicals can be directly...