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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 the temperature decreases, the sensitivity of the conductivity to the gate voltage, α ≡ 1/σdσ/dVg, increases in the range from 300 to 80 K and decreases sharply below 80 K, where the nonlinear conductivity begins to depend on Vg. The results can be explained by the formation of an electronic crystal at low temperatures.

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) techni ...

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 quasi-two-dimensional nature of TMDCs; a different CDW symmetry is unexpected. We report herein the successful creation of pristine single-layer VSe$_2$, which shows a ($\sqrt7 \times \sqrt3$) CDW in contrast to the (4 $\times$ 4) CDW for the layers in bulk VSe$_2$. Angle-resolved photoemission spectroscopy (ARPES) from the single layer shows a sizable ($\sqrt7 \times \sqrt3$) CDW gap of $\sim$100 meV at the zone boundary, a 220 K CDW transition temperature twice the bulk value, and no ferromagnetic exchange splitting as predicted by theory. This robust CDW with an exotic broken symmetry as the ground state is explained via a first-principles analysis. The results illustrate a unique CDW phenomenon in the two-dimensional limit.

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 that TP0 is the temperature of the formation of this CDW. The charges condensed in this CDW show relatively high density and, at the same time, extremely low mobility. Their mobility appears low in the single-particle states as well, giving a plausible clue to the surprising dielectric-like temperature variation of σ above TP0.

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 distinct phases associated with germanene, the analog of graphene made of germanium (Ge) instead of carbon, can be grown on Ag(111) as observed by scanning tunneling microscopy, low-energy electron diffraction, and angle-resolved photoemission spectroscopy. One such germanene exhibits an atom-resolved alternatively buckled full honeycomb lattice, which is tensile strained and partially commensurate with the substrate to form a striped phase (SP). The other, a quasifreestanding phase (QP), is also consistent with a honeycomb lattice with a lattice constant incommensurate with the substrate but very close to the theoretical value for freestanding germanene. The SP, with a lower atomic density, can be driven into the QP and coexist with the QP by additional Ge deposition. Band mapping and first-principles calculations with proposed SP and QP models reveal an interface state exists only in the SP but the characteristic \ensuremath{\sigma} band of freestanding germanene emerges only in the QP---this leads to an important conclusion that adlayer-substrate commensurability plays a key role to affect the electronic structure of germanene. The evolution of the dual germanene phases manifests the competitive formation of Ge-Ge covalent and Ge-Ag interfacial bonds.

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 methane as the source and molten gallium (Ga) as catalysts. The very low temperature graphene synthesis is made possible by carbon attachment to the island edges of pre-existing graphene nuclei islands, and causes no damages to the substrates. A key benefit of using molten Ga catalyst is the enhanced methane absorption in Ga at lower temperatures; this leads to a surprisingly low apparent reaction barrier of ~0.16 eV below 300 °C. The faster growth kinetics due to a low reaction barrier and a demonstrated low-temperature graphene nuclei transfer protocol can facilitate practical direct graphene synthesis on many kinds of substrates down to 50–100 °C. Our results represent a significant progress in reducing graphene synthesis temperature and understanding its mechanism.

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. Here we report a detailed method to conduct reliable STM and STS on FSG with atomic precision. We found that FSG is intrinsically rippled and exhibits a nonlinear strain-stress relation under applied normal forces; it shows a very soft region of bending strain and stiffer regions of in-plane tensile strain once the nanoscale ripples of FSG are eliminated. The elimination of the nanoripples can be controlled by tip-induced pulling or pushing force through the so-called closed-loop Z-V STS mode which can monitor the FSG deformation. A key factor for controllable STM and STS measurements is to select tunneling set points to place FSG in metastable configurations, as determined from stress-strain (i.e., Z-V) response. Atomic imaging and electronic states thus measured must be interpreted by considering the dynamical deformation of FSG as tunneling parameters, and therefore tip-FSG forces, are varied.

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 deformations of a rippled FSG involve sign reversals of the nanoripple’s local curvatures, termed “flipping”. In contrast to elastic deformation of graphene, flipping of a FSG nanoripple encounters a stress barrier and therefore makes a rippled FSG metastable, as evidenced by monitoring a yielding process in both experiments and molecular dynamics simulations. The evolution of nanoripples subjected to STM control is also fully consistent with atom-resolved images. Our work therefore shows how rippled 2D carbon deforms at nanoscale.

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 the porous body. In contrast, the other with a mesopore diameter of ∼3 nm has a distinct distribution of crystalline Pt nanoparticles on the outermost surfaces of the porous material and extremely small (

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-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), fluorescence optical microscopy (FOM), photoluminescence spectroscopy (PL), photoluminescence excitation spectroscopy (PLE), as well as time-of-flight (TOF) and transient electroluminescence (TrEL) techniques. The mobility is about two orders of magnitude greater for drop-cast films than for their spin-cast counterparts. Drop-casting in the presence of a vertical static electric field (E-casting) results in films with an additional increase in mobility of about one order of magnitude. While PL and PLE spectra vary with the method of film preparation, there is no correlation between emission spectra and charge-carrier mobility. Our XRR measurements on spin-cast films indicate layering along the film depth while no such structure is found in drop-cast or E-cast films, whereas FESEM examination indicates that nanodomains within drop-cast films are eliminated in the E-cast case. These observations indicate that carrier transport is influenced by structure on two different length scales. The low mobility observed in spin-cast films is a direct result of a global layered structure with characteristic thickness of ca. 4 nm: in the absence of this layered structure, drop-cast films with inherent nanoscale heterogeneities (ca. 20 nm in size) exhibit much better hole mobility. Elimination of nanodomains via electric-field alignment results in further improved charge mobility.

We report a scanning tunneling microscopy (STM) study of potassium ( K )-doped C 60 ( K x C 60) 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 C 60-induced substrate reconstruction was also observed. On Ag (111), K -doping induces C 60 reorientation and results in a p(2 × 2) superstructure. All observed superstructures suggest intrinsic inhomogeneity of K -doping in fullerene materials, which must be further understood.

This paper reports an experimental observation of the square root q behavior of $\ensuremath{\pi}$-plasmons in free-standing graphene. The authors used very high resolution EELS measurements to be able to probe plasmonic excitations in the long wavelength limit.

We demonstrate that a C60 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 C60/Co interfacial magnetic anisotropy that we have measured quantitatively in situ as a function of the C60 coverage. Comparison with state-of-the-art ab initio calculations show that this interfacial anisotropy mainly arises from the local hybridization between C60 pz and Co dz2 orbitals. By generalizing these arguments, we also demonstrate that the hybridization of C60 with a Fe(110) surface decreases the perpendicular magnetic anisotropy. These results open the way to tailor the interfacial magnetic anisotropy in organic-material-ferromagnet systems.

Fullerene C60 and C70 mixture films on Ag(100) have been studied with scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). In contrast to the recently found aperiodic incommensurate "(111)" phase of C60 on Ag(100), C70 formed a commensurate c(6 ×4) structure. Co-deposited C60/C70 mixture layers were examined as a function of controlled C60/C70 ratio. LEED showed that overall film symmetry evolved from "(111)" to c(6 ×4) as C60 density increased. In contrast, STM revealed that C60 and C70 distributions were highly non-uniform, with many prominent unidirectional wavy bright lines due to local C60 aggregation.

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 of various adsorption structures, conformations and surface reconstructions. Here we present two cases of C 60 adsorbed on Ag(1 0 0) and Cu(1 1 1) that illustrate the importance of competitive interactions in the presence of adsorbate-induced reconstruction. In both studies, strong STM contrasts derive from topographic features arising from reconstructed substrates. C 60 /Ag(1 0 0) presents a unique uniaxially incommensurate molecular packing. We have also found that one can control molecular ordering with a stepped surface to produce a single-domain film. In C 60 /Cu(1 1 1), we are able to obtain metastable, yet well-defined, molecular ordering with precise annealing procedures. These metastable states exhibit a clear correlation between molecular contrast patterns and the adlayer rotation angle, as a consequence of competitive interactions between optimizing C 60 bonding at preferred reconstructive sites and C 60 –C 60 repulsion. Finer control of selective preparation of these metastable structures offers a plausible way of fabricating nanostructures by design.

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 nucleation barrier as ?eV.

The diffusion and coarsening of two-dimensional homoepitaxial islands on Cu(100) and Ag(100) surfaces have been studied at room temperature with time-sequenced scanning tunneling microscopy. Quantitative analyses of the dependence of island diffusion coefficient $D$ vs the island side length $L$, $D\ensuremath{\propto}{L}^{\ensuremath{-}\ensuremath{\alpha}}$, yield noninteger scaling exponents which are consistent with island coalescence. Moreover, the near absence of island decay shows that the island diffusion occurs via mass transport along the island periphery.

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 devices show similar inferior reliability. For rough NMOS devices, as compared to flat NMOS devices, the Weibull plot of T/sub BD/ shows a 4.9-fold enhancement at 63% failure rate. The time evolutions of the light emission from rough PMOS and NMOS diodes degrade much less than those of flat PMOS and NMOS diodes. The momentum reduction perpendicular to the Si/SiO/sub 2/ interface by roughness scattering could possibly make it difficult to form defects in the bulk oxide and at the Si/SiO/sub 2/ interface by the impact of the energetic electrons and holes.

The oxide roughness of metal-oxide-silicon diodes can be intentionally controlled by the very high vacuum pre-bake and the growth conditions during rapid thermal oxidation. Both surface and Si/oxide interface have the similar magnitude of roughness measured by atomic force microscopy, indicating the conformal growth of oxide. At accumulation bias (positive gate bias), the holes tunnel from gate electrode to n-type Si through the ultrathin oxide, and recombine with the electrons in the accumulation region radiatively if phonon scattering and roughness scattering provide the necessary momentum. The light emission intensity increases with increasing oxide roughness. Strong electroluminescence with an external quantum efficiency of ~ 2×10-6 at room temperature was observed from a rough metal-oxide-silicon tunneling diode.

This study investigates the surface chemistry and the ordering characteristics of coadsorbed hydrogen and chlorine atoms, generated by the exposure of the Si100 surface to gas-phase HCl molecules at various substrate temperatures, by scanning tunneling microscopy STM, core-level photoemission spectroscopy, and Monte Carlo simulation. Experimental results show that saturation exposure to HCl causes all surface dangling bonds to be terminated by the two fragments H and Cl atoms and that the number of H-terminated sites exceeds that of Cl-terminated ones by more than 10%. This finding suggests that, in addition to the dominant dissociative chemisorption, atomically selective chemisorption or atom abstraction occurs. STM images reveal that some Cl-terminated sites form patches with a local 22 structure at 110 K and that the degree of ordering is reduced as the substrate temperature increases. Results of Monte Carlo simulations demonstrate the importance of including dissociative fragment-adsorbate interactions during the random adsorption of diatomic molecules. Comparing the correlations between Cl-terminated sites identified from STM images and those predicted by simulation reveals two effective interaction energies of 8.52.0 and 3.52.0 meV between a dissociative fragment Cl atom and a nearest neighboring Cl adsorbates in the same dimer row and in the adjacent row, respectively.

In this work, we report the development of a MEMS electrostatic resonator used as the actuator for a fast-scan STM system (FS-STM). The FS-STM requires an actuator which can oscillate with a relatively large amplitude (>3 μm) at a resonant frequency of larger than 10 kHz, which cannot be achieved by using piezoelectric actuators. Under ambient pressure, the measurement results show that the oscillating amplitude of the proposed electrostatic actuator is about 3.1 μm at 11.8 kHz. This resonator will be used to generate the scanning motion of the tip in lateral direction, which might significantly improve the scanning rate of a typical radio-frequency STM.

We theoretically predict the occurrence of negative differential resistance (NDR) in scanning tunneling spectroscopy for a pure ${\text{C}}_{60}$ 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 be reduced if an oxidized or Cu-terminated tip is used. A detailed decomposition of the total tunneling current into its contributions from individual molecular orbitals reveals that only some of the orbitals on the tip and on the ${\text{C}}_{60}$ can be ``matched up'' to give a contribution to the current and that the NDR is a consequence of the mismatch between these specific orbitals within particular ranges of bias voltage. Moreover, the NDR characteristics, including the peak positions and the peak-to-valley ratios, are found to depend on the tip material, tip geometry, and tip-to-molecule position.

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 superlattice structures which we interpret as the ordering of Na (vacancy) clusters. These results lead to a picture of coexisting local moments and itinerant carriers and form the first step towards understanding the many anomalous properties of cobaltates.

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 ∼573 K to ∼973 K. This trend contradicts the prediction in conventional nucleation theory. At low and high temperatures, C60 commence nucleation on both sides of surface steps in a "bi-directional step flow" mode. However, anisotropy occurs within an intermediate temperature range, in which C60 nucleate predominantly at upper step edges. Surprisingly, in-situ growth measurements at this intermediate temperature range revealed that C60 actually start nucleating from lower step edges, with concomitant formation of Ni terraces underneath. These anomalous thermal dependence of diffusivity and the peculiar growth morphology of C60 on Ni(111) are attributed to C60-induced reconstruction of Ni(111) at higher temperature.

Using scanning tunneling microscopy and spectroscopy and first principles calculations, we have systematically studied the morphological and electronic structures of a ${\mathrm{C}}_{60}$ monolayer on Ag(100). Our results reveal that the bright-dim contrast has a definite geometric origin and there are two types of dim ${\mathrm{C}}_{60}$ molecules, one a monomer and the other a dimer. With distinctive electronic properties and distinctive charge transfers in the differently adsorbed ${\mathrm{C}}_{60}$ molecules, this ${\mathrm{C}}_{60}$ monolayer is highly inhomogeneous and aperiodic.

Detailed combined scanning tunneling microscopy and low-energy electron diffraction measurements reveal that the structure of a C 6 0 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-range order, and room-temperature fluctuations of the contrast show that a thermal equilibrium state is reached. The nature of this controversial bright-dim C 6 0 contrast is clarified as a topographic feature due to C 6 0 -induced reconstruction underneath the dim C 6 0 molecules. Due to interactions between the incommensurate C 6 0 adlayer and the reconstructed substrate, the (111) phase is distorted laterally, forming a novel "tetramer" configuration of specific contrast order. The spatial distribution of these tetramers is aperiodic; this has crucial implications for the peculiar short-range contrast order observed experimentally. A lattice gas model with anisotropic nearest-neighbor interactions and a configuration energy of the tetramer is developed. Quantitative agreements between observation and modeling are achieved with reasonable phenomenological parameters derived within experimental constraints.

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 ${\mathrm{C}}_{60}$ contrast to an interfacial restructuring beneath each dim ${\mathrm{C}}_{60},$ with its ordering determined by competing interactions in the reconstructed interface and of the rhombus configuration.

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 between successive detachments. Upon oxygen adsorption on vicinal Ag(110), a dramatic change in step configuration occurs. Steps which are originally equi-spaced due to repulsive step-step interactions are compressed into bunches. This process is driven by the formation of large (110) facets, on which oxygen atoms arrange into chains along [001]. The faceting dynamics are sensitive to the orientation of the step edge: proceeding by nucleation for close-packed steps and by spinodal decomposition for steps at an acute angle to the oxygen chains. A closer inspection of these oxygen chains reveals that each incorporates an additional row of silver atoms. When oxygen dosing pressures are kept below 10-6 Torr, silver atoms detaching from the step edge provide a sufficient supply necessary for the formation of the added row. With higher oxygen pressures, the silver atoms required for the oxygen chains are extracted directly from the terraces, resulting in the formation of large, long-lived etch pits.

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 (>1.3μm) of InAs quantum dots capped with sequential GaAs∕InAs binary growth is mainly due to a weaker quantum confinement effect. This particular capping method is better than the traditional molecular beam epitaxy with simultaneous In∕Ga∕As deposition, and much better than a capping method with separated Ga deposition followed by As and InAs growth.

We have successfully used scanning tunneling microscope-assisted chemical vapor deposition to fabricate magnetic nanostructures as fine as 5 nm wide and