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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. Another frequently observed defect type is identified as arising from an interstitial Sn atom; this defect can be switched to a more stable configuration consisting of a Sn substitutional defect + P adatom, by application of an electrical pulse via the STM tip. DFT calculations show that this pulse-induced structural transition switches the system from a non-magnetic configuration to a magnetic one. We introduce States Projected Onto Individual Layers (SPOIL) quantities which provide information about atom-wise and orbital-wise contributions to bias-dependent features observed in STM images., We acknowledge the Indo-French Centre for the Promotion of Advanced Scientific Research (CEFIPRA) for funding our collaboration. We are especially grateful for assistance provided by the Late Ms. A. Sathidevi. We are grateful to the ANR and MOST (DEFINE2D Project No. ANR-20-CE09-0023, MOST 110-2923-M-002-010) for financial support and the TUE-CMS, JNCASR, and the Sheikh Saqr Laboratory, JNCASR, for computational resources. D.S. acknowledges support from the NPDF of the Science and Engineering Research Board (SERB), Government of India.

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 trapped in a potential well on the MoS2 surface due to its wave nature. Because the relaxed electron and the resonant electron have the same spin, through the action of the Pauli exclusion principle, the lifetimes of the resonant electrons can be substantially prolonged when the relaxed electrons engage in resonance trapping. The linewidth of the FER is thus considerably reduced to as narrow as 12 meV. The coexistence of the resonant electron and the relaxed electron requires the emission of two electrons, which can occur through the exchange interaction.

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 a quasifreestanding phase (QP). Using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM), we demonstrate that the 2D monatomic Pb layer, instead of being on top, grows side-by-side with QP germanene, not only driving the SP-to-QP transition but further enhancing the lattice order of QP as revealed by the dramatic evolution of LEED-spot shape and uniform STM-imaged array of moir\'e patterns. Originally oriented at 30\ifmmode^\circ\else\textdegree\fi{} to Ag(111), the QP germanene transits first to a narrow distribution of rotations then to two sharp domains at \ifmmode\pm\else\textpm\fi{}3\ifmmode^\circ\else\textdegree\fi{} from 30\ifmmode^\circ\else\textdegree\fi{}, while the monoatomic Pb domains twist from \ifmmode\pm\else\textpm\fi{}4.3\ifmmode^\circ\else\textdegree\fi{} to \ifmmode\pm\else\textpm\fi{}9.5\ifmmode^\circ\else\textdegree\fi{} with respect to Ag(111). The analysis of higher-order coincidence for both layers on Ag(111) is in complete agreement with LEED results and reveals a picture of two insoluble monatomic layers driving each other to a more optimal configuration. Moreover, the corresponding density of states measured by scanning tunneling spectroscopy for quality-enhanced QP germanene exhibits a clear V-shape distribution at the energy \ensuremath{\sim}0.4 eV above Fermi level, indicating the Fermi-Dirac cone, which agrees with our density functional theory calculation.

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 studies of the Seebeck coefficient $S$ of the quasi-one-dimensional compound $\mathrm{Nb}{\mathrm{S}}_{3}$, together with the temperature dependence of its specific conductivity ${\ensuremath{\sigma}}_{\mathrm{s}}$ and heat capacity ${c}_{p}$. The monoclinic phase $(\mathrm{Nb}{\mathrm{S}}_{3}$-II) is studied over the temperature range $T=80--400$ K, which covers two charge density wave (CDW) transitions at ${T}_{\mathrm{P}1}=360$ K (CDW-1) and ${T}_{\mathrm{P}2}=150$ K (CDW-2). The $S(T)$ curves show features in the vicinities of both CDW transitions and appear to be correlated with the value of ${\ensuremath{\sigma}}_{\mathrm{s}}(300\phantom{\rule{0.28em}{0ex}}\mathrm{K})$: The increase of $S$ below ${T}_{\mathrm{P}1}$ in the high-Ohmic samples reveals a complete dielectrization of the electronic spectrum, while in the low-Ohmic samples $S$ decreases below ${T}_{\mathrm{P}1}$ and even becomes negative below ${T}_{\mathrm{P}2}$. The magnitude of $S$ in low-Ohmic samples at $Tl{T}_{\mathrm{P}2}$ is well below ${k}_{\mathrm{B}}/e\ensuremath{\approx}86\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}/\mathrm{K},{k}_{B}$ being the Boltzmann constant and $e$ the elementary charge, which is surprisingly low for a usual CDW semiconducting state. Our results suggest that at ${T}_{\mathrm{P}1}$ the main electronic band with $p$-type carriers becomes gapped, while some $n$-type carriers can remain in a separate band with low density of states. These carriers, whose concentration is defined by the compositional doping, are gapped at ${T}_{\mathrm{P}2}$. We also report $S$ for the triclinic dielectric phase of $\mathrm{Nb}{\mathrm{S}}_{3}\phantom{\rule{4pt}{0ex}}(\mathrm{Nb}{\mathrm{S}}_{3}\text{\ensuremath{-}}\mathrm{I})$. Its low absolute value, $\ensuremath{\sim}{k}_{\mathrm{B}}/e$, and the anomalous temperature dependence demonstrate that $\mathrm{Nb}{\mathrm{S}}_{3}\text{\ensuremath{-}}\mathrm{I}$ is neither a semiconductor nor a CDW conductor in the usual sense.

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 ...

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, ${\mathrm{Ag}}_{2}\mathrm{Pb}$ and ${\mathrm{Ag}}_{2}\mathrm{Ge}$, with $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R{30}^{\ensuremath{\circ}}$ and $\frac{19}{20}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\frac{19}{20}\sqrt{3}R{30}^{\ensuremath{\circ}}$ structures on Ag(111) but the surface electronic structures exhibit a most evident difference at the Ag(111) surface zone boundary ${\overline{M}}_{\mathrm{Ag}(111)}$, where the single band and the splitting ones were observed, respectively. Up to 1 ML, ${\mathrm{Ag}}_{2}\mathrm{Ge}$ subsequently develops into germanene with a striped phase and then a quasifreestanding phase, as previously reported [Lin et al., Phys. Rev. Mater. 2, 024003 (2018)], while ${\mathrm{Ag}}_{2}\mathrm{Pb}$ evolves to a dense Pb(111) phase that also reveals splitting bands at ${\overline{M}}_{\mathrm{Ag}(111)}$. We discover that the larger (smaller) atomic size of a Pb (Ge) atom with respect to an Ag atom causes the commensurate (incommensurate) interfaces and further demonstrate that the splitting bands of ${\mathrm{Ag}}_{2}\mathrm{Ge}$ surface alloy and 1-ML Pb film originated from the commonly incommensurate interface with Ag(111), which mediates umklapp scattering by inducing the mirror image of the pristine ${\mathrm{Ag}}_{2}\mathrm{Ge}$ and Pb(111) bands relative to ${\overline{M}}_{\mathrm{Ag}(111)}.$

We report studies of the Seebeck coefficient S of the quasi-one-dimensional compound NbS3, together with the temperature dependence of its specific conductivity ss and heat capacity c(p). The monoclinic phase (NbS3-II) is studied over the temperature range T = 80-400 K, which covers two charge density wave (CDW) transitions at T-P1 = 360 K (CDW-1) and T-P2 = 150 K (CDW-2). The S(T) curves show features in the vicinities of both CDW transitions and appear to be correlated with the value of sigma(s)(300 K): The increase of S below T-P1 in the high-Ohmic samples reveals a complete dielectrization of the electronic spectrum, while in the low-Ohmic samples S decreases below T-P1 and even becomes negative below T-P2. The magnitude of S in low-Ohmic samples at T < T-P2 is well below k(B)/e approximate to 86 mu V/K, k(B) being the Boltzmann constant and e the elementary charge, which is surprisingly low for a usual CDW semiconducting state. Our results suggest that at T-P1 the main electronic band with p-type carriers becomes gapped, while some n-type carriers can remain in a separate band with low density of states. These carriers, whose concentration is defined by the compositional doping, are gapped at T-P2. We also report S for the triclinic dielectric phase of NbS3 (NbS3-I). Its low absolute value, similar to k(B)/e, and the anomalous temperature dependence demonstrate that NbS3-I is neither a semiconductor nor a CDW conductor in the usual sense.

The basic and the charge density wave (CDW) structures of the monoclinic ${\mathrm{NbS}}_{3}$-II polymorph were studied by synchrotron x-ray diffraction, ab initio calculations, simulation of electron diffraction patterns, and by atomic-resolution transmission electron and low-temperature scanning tunneling microscopies. It is confirmed that the basic structure belongs to the space group $P{2}_{1}/m$ and is described with a unit cell, formed of four pairs of symmetry-related trigonal prismatic (TP) columns [${a}_{0}=0.96509(8)\phantom{\rule{4pt}{0ex}}\mathrm{nm}$, ${b}_{0}=0.33459(2)\phantom{\rule{4pt}{0ex}}\mathrm{nm}$, ${c}_{0}=1.9850(1)\phantom{\rule{4pt}{0ex}}\mathrm{nm}$, and ${\ensuremath{\beta}}_{0}=110.695{(4)}^{\ensuremath{\circ}}$]. The incommensurate components of the two CDWs, $\stackrel{P\vec}{{q}_{1}}=(0,\phantom{\rule{4pt}{0ex}}0.298,0)$ and $\stackrel{P\vec}{{q}_{2}}=(0,\phantom{\rule{4pt}{0ex}}0.352,\phantom{\rule{4pt}{0ex}}0)$, are related as ${q}_{{1}_{b}}+2{q}_{{2}_{b}}\ensuremath{\approx}1$. Both CDWs form their own modulation patterns with unit cells (${a}_{\mathrm{m}}=2{a}_{0}$, ${b}_{\mathrm{m}}={b}_{0}/{q}_{\mathrm{jb}}$, ${c}_{\mathrm{m}}={c}_{0}$, ${\ensuremath{\beta}}_{m}={\ensuremath{\beta}}_{0}$) and are ordered along adjacent isosceles TP columns either pairwise or with both columns modulated by either the $\stackrel{P\vec}{{q}_{1}}$ or $\stackrel{P\vec}{{q}_{2}}$ CDW only. The CDWs are ordered according to one of the two possible modulation pattern space groups, $Cm$ or $C2/m$. If considered as long-period commensurate, the entire modulated structure with both CDWs included is described within experimental error with an enlarged unit cell ($a=2{a}_{0}$, $b=37{b}_{0}$, $c={c}_{0}$, and $\ensuremath{\beta}={\ensuremath{\beta}}_{0}$) and with all atoms displaced from their average positions in accord with the specified modulation pattern.

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.

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. Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy and spectroscopy, we observed the emergence of a (2 × 2) charge density wave order in single-layer TiTe2 with a transition temperature of 92 ± 3 K. Also observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K. Surprisingly, no charge density wave transitions were observed in two-layer and multi-layer TiTe2, despite the quasi-two-dimensional nature of the material in the bulk. The unique charge density wave phenomenon in the single layer raises intriguing questions that challenge the prevailing thinking about the mechanisms of charge density wave formation.Due to reduced dimensionality, the properties of 2D materials are often different from their 3D counterparts. Here, the authors identify the emergence of a unique charge density wave (CDW) order in monolayer TiTe2 that challenges the current understanding of CDW formation.

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.

A systematic study of polycrystalline $1T\ensuremath{-}{\mathrm{TiSe}}_{2\ensuremath{-}\ensuremath{\delta}}$ with controlled Se loss indicates that the unconventional charge density wave (CDW) phase is found to be most pronounced in samples with $\ensuremath{\delta}\ensuremath{\sim}0.12$, instead of being Se vacancy free. The level of Se vacancy defects and temperature determines whether $1T\ensuremath{-}{\mathrm{TiSe}}_{2\ensuremath{-}\ensuremath{\delta}}$ should be categorized as a semiconductor, a semimetal, or an excitonic insulator. An interpretation using a general band picture of $p$-type doped narrow-band-gap semiconductor with an impurity band (IB) in proximity to the valence band (VB) is proposed to explain the evolution of electronic structures for $1T\ensuremath{-}{\mathrm{TiSe}}_{2\ensuremath{-}\ensuremath{\delta}}$, from the intermediate doping of $\ensuremath{\delta}\ensuremath{\sim}0.08$, to the critical doping of $\ensuremath{\delta}\ensuremath{\sim}0.12$ showing an anomalous resistivity peak between $\ensuremath{\sim}100--200\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, and to the heavily doped of $\ensuremath{\delta}\ensuremath{\sim}0.17$ as an $n$-type degenerate semiconductor. Integrated chemical analysis and physical property characterization, including electron probe microanalysis (EPMA), synchrotron x-ray diffraction, resistivity, and Seebeck coefficient measurement results are provided for the polycrystalline samples prepared via vacuum-sealed high temperature annealing route.

We review the features of the charge density wave (CDW) conductor $\mathrm{Nb}{\mathrm{S}}_{3}$ (phase II) and include several additional results from transport, compositional, and structural studies. Particularly, we highlight three central results: (1) In addition to the previously reported CDW transitions at ${T}_{\mathrm{P}1}=360\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{P}2}=150\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, a third CDW transition occurs at a much higher temperature ${T}_{\mathrm{P}0}\ensuremath{\approx}620\ensuremath{-}650\phantom{\rule{0.16em}{0ex}}\mathrm{K}$; evidence for the nonlinear conductivity of this CDW is presented. (2) We show that the CDW associated with the ${T}_{\mathrm{P}2}$ transition arises from S vacancies acting as donors. Such a CDW transition has not been observed before. (3) We demonstrate the exceptional coherence of the ${T}_{\mathrm{P}1}$ CDW at room temperature. The effects of uniaxial strain on the CDW transition temperature and transport are reported.

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.

In this study, we found that the local density of states and ionization energy spectrum of the valence electrons of methylnitrene (NCH3) adsorbed on Cu(110) can be calculated from molecular orbital calculations of a simple artificial isolated NCH3–Cu2 molecular cluster in which the two Cu atoms form bonds to the N atom. Such a NCH3–Cu2 cluster represents the basic structural unit of a NCH3 molecule adsorbing on a Cu double-added-row structure. This finite NCH3–Cu2 cluster structure is not optimized as a single system but is extracted directly from an optimized surface structure obtained by density functional theory with periodic boundary conditions. With this approach, we obtained excellent agreement between the measured ultraviolet photoemission spectra (UPS) and the theoretical calculation results. To further examine this minimal cluster concept, we analyzed methoxy (OCH3) adsorption on Cu(110) and found a OCH3–Cu3 cluster structure. On the basis of this structure, we calculated UPS and also obtained su...

In this study, we used scanning tunneling microscopy (STM) and density functional theory (DFT) to examine the bonding structure of CH3N adsorbed on the Cu(110) surface. A previous study [Chin. J. Phys.2005, 43, 212–218] shows the adsorbed CH3N aggregate to form a zigzag structure with a p(2 × 3) unit cell, without considering the possibility of adsorbate-induced surface reconstruction. Here, we propose a revised adsorption structure, with the key feature of bonding each CH3N with two Cu adatoms in a tetrahedral manner. Three structure models (double-row, dimer, and alternative-dimer) are examined by ab initio calculations. We find that the most energetically favorable model is the double-row model with CH3N bonding alternatingly along either side of double added rows from Cu adatoms.

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 (

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 rise to an upward tilted Pt–Oa configuration with the Oa singly connected to one of the underlying carbon (C1L) and leaving a semifilled and lone-paired orbital on the other carbon (C*). The highly reactive and long-lived C* plays an important role determining the energy barriers and branching nature of the subsequent reaction pathways. By being long-lived, reactions of the C* spilt into the early- and late-C* pathways depending when the long-lived C* is participated in the reaction. The early-C* pathways involve direct reactions of the C* with either the Pt or the neighboring carbons with low energy barriers (Ea < 0.3 eV) and give rise to stable local minimum st...

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 series, we found that CO oxidation on a Au-enriched Ag(110) surface leads to an exponential depletion of oxygen with time and a reaction rate that is synergistically enhanced by the presence of Au. First principles calculations indicate that the local atomic configuration around the active reaction sites at the chain ends and the preference of Au atom substitution into the subsurface second Ag layer are of critical importance. By calculating CO adsorption energies and reaction barriers for plausible reaction pathways, a detailed description of the CO oxidation reaction emerges. For the optimal reaction pathway, a large (∼0.09 eV) barrier reduction and a small b...

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.

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

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 substrate reconstruction was also observed. On Ag(111), K-doping induces C60 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.

Formation of two-dimensional islands of methyl radical (CH3) on Cu(111) and Cu(110) surfaces, and methylnitrene radical (NCH3) islands on Cu(110) 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 (√3 ×√3)-R30° island structure that was more readily observed at T

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 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 CH 3 (ads) molecules tend to aggregate into (√3 x √3) R30° structure in 2D islands at any coverage up to one saturation monolayer. The island sizes depend on the amount of gaseous exposure, and at a given exposure, the sizes are found to decrease when the surface temperature is raised to near the reaction temperature of CH 3 (ads). The adsorbate islanding behavior can be correlated with the first-order reaction kinetics in the generation of long-chain alkene products detected in the thermal desorption. From this and other related studies on methyl-solid interactions, we infer that the islanding phenomenon may be quite general in catalytic systems.

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 to study the temperature evolution of C60 contrast. We clearly show that the bright C60 turns into the dim C60, starting at approximately ∼280 K. This phenomenon, along with the simulated contrast evolution of multi-layer C60 film and the molecular resolved non-contact AFM (NCAFM) measurements, are consistent with a topographic interpretation of the bright–dim contrast, in contrary to what is previously believed.

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 step-edge line tension.