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One-dimensional (1D) transition-metal chalcogenides (TMCs) are attracting increasing scientific and technological interest, especially for ultrasmall electronic interconnects and highly active cata...

Mixed-dimensional van der Waals heterostructures are scientifically important and practically useful because of their interesting exotic properties resulting from their novel hybrid structures. This study reports the composition- and phase-selective fabrication of low-dimensional molybdenum/tellurium (Mo/Te) compounds and the direct synthesis of mixed-dimensional in-plane 1D-2D Mo

The ability to improve and to modulate the heterogeneous charge transfer kinetics of two-dimensional (2D) semiconductors, such as MoS2, is a major challenge for electrochemical and photoelectrochemical applications of these materials. Here we report a continuous and reversible physical method for modulating the heterogeneous charge transfer kinetics at a monolayer MoS2 working electrode supported on a SiO2/p-Si substrate. The heavily doped p-Si substrate serves as a back gate electrode; application of a gate voltage (VBG) to p-Si tunes the electron occupation in the MoS2 conduction band and shifts the conduction band edge position relative to redox species dissolved in electrolyte in contact with the front side of the MoS2. The gate modulation of both charge density and energy band alignment impacts charge transfer kinetics as measured by cyclic voltammetry (CV). Specifically, cyclic voltammograms combined with numerical simulations suggest that the standard heterogeneous charge transfer rate constant (k0...

The coexistence of metallic and semiconducting polymorphs in transition-metal dichalcogenides (TMDCs) can be utilized to solve the large contact resistance issue in TMDC-based field effect transistors (FETs). A semiconducting hexagonal (2H) molybdenum ditelluride (MoTe

Mixed-dimensional van der Waals heterostructures are scientifically important and practically useful because of their interesting exotic properties resulting from their novel hybrid structures. This study reports the composition- and phase-selective fabrication of low-dimensional molybdenum/tellurium (Mo/Te) compounds and the direct synthesis of mixed-dimensional in-plane 1D-2D Mo6 Te6 -MoTe2 heterostructures. The composition and phase of the Mo/Te compounds are controlled by changing the Te atomic flux that is adjusted by the Te temperature. Metallic 1D Mo6 Te6 wires with an intrinsic 1D structure with a diameter of 3-8 nm and length of 100-300 nm are synthesized to form wire networks under low Te flux conditions, whereas the semiconducting few-layer 2H MoTe2 films preferentially oriented along the direction are obtained under high Te flux. Under medium Te flux, the mixed-dimensional in-plane 1D-2D Mo6 Te6 -MoTe2 heterostructures are synthesized in which the semiconducting few-layer 2H MoTe2 circular domains are edge-contacted by the metallic 1D Mo6 Te6 wire networks. Furthermore, the present Te-flux-controlled method reveals that the 1D Mo6 Te6 networks change to few-layer MoTe2 films as the Te flux increases. The in-plane 1D-2D Mo6 Te6 -MoTe2 heterostructures synthesized by this method can be considered as advanced edge-contacted 2D semiconductors for high-performance 2D electronics.

Heteroepitaxy between transition-metal dichalcogenide (TMDC) monolayers can fabricate atomically thin semiconductor heterojunctions without interfacial contamination, which are essential for next-generation electronics and optoelectronics. Here we report a controllable two-step chemical vapor deposition (CVD) process for lateral and vertical heteroepitaxy between monolayer WS2 and MoS2 on a c-cut sapphire substrate. Lateral and vertical heteroepitaxy can be selectively achieved by carefully controlling the growth of MoS2 monolayers that are used as two-dimensional (2D) seed crystals. Using hydrogen as a carrier gas, we synthesize ultraclean MoS2 monolayers, which enable lateral heteroepitaxial growth of monolayer WS2 from the MoS2 edges to create atomically coherent and sharp in-plane WS2/MoS2 heterojunctions. When no hydrogen is used, we obtain MoS2 monolayers decorated with small particles along the edges, inducing vertical heteroepitaxial growth of monolayer WS2 on top of the MoS2 to form vertical WS2/MoS2 heterojunctions. Our lateral and vertical atomic layer heteroepitaxy steered by seed defect engineering opens up a new route toward atomically controlled fabrication of 2D heterojunction architectures.

The ability to improve and to modulate the heterogeneous charge transfer kinetics of two-dimensional (2D) semiconductors, such as MoS

International audience; have revealed the simultaneous presence of both high room temperature hole mobility above 10 cm 2 V −1 s −1[5] and, crucially , a negative temperature exponent for the mobility, i.e., mobility increases as temperature decreases down to about 180 K. [6] These observations and others, including mobility anisotropy [7,8] and a robust Hall effect, [9,10] are strong evidence for band-like transport in rubrene (band-width ≈0.5 eV), [11] where many features of classical band transport are observed. However, the estimated carrier mean free paths remain on the order of the unit cell dimension (≈1 nm), thus precluding the typical band picture in conventional semiconductors. The precise nature of band-like transport in organic semiconductor crystals is still being explored. [12] Currently, a consistent picture of charge transport in high mobility crystals is offered by the dynamic disorder model (DDM, also known as transient localization), in which the carrier transient localization length l loc and the propagation rate ω are determined by a competition between intermolecular electronic coupling and charge-phonon interaction ; in this scenario the carrier mobility μ can be evaluated as [13] µ ω = B loc 2 e k T l (1) Isotopic substitution is a useful method to study the influence of nuclear motion on the kinetics of charge transport in semiconductors. However, in organic semiconductors, no observable isotope effect on field-effect mobility has been reported. To understand the charge transport mechanism in rubrene, the benchmark organic semiconductor, crystals of fully isotopically substituted rubrene, 13 C-rubrene (13 C 42 H 28), are synthesized and characterized. Vapor-grown 13 C-rubrene single crystals have the same crystal structure and quality as native rubrene crystals (i.e., rubrene with a natural abundance of carbon isotopes). The characteristic transport signatures of rubrene, including room temperature hole mobility over 10 cm 2 V −1 s −1 , intrinsic band-like transport, and clear Hall behavior in the accumulation layer of air-gap transistors, are also observed for 13 C-rubrene crystals. The field-effect mobility distributions based on 74 rubrene and 13 C-rubrene devices, respectively, reveal that 13 C isotopic substitution produces a 13% reduction in the hole mobility of rubrene. The origin of the negative isotope effect is linked to the redshift of vibrational frequencies after 13 C-substitution, as demonstrated by computer simulations based on the transient localization (dynamic disorder) scenario. Overall, the data and analysis provide an important benchmark for ongoing efforts to understand transport in ordered organic semiconductors.

Controlling the growth of two-dimensional (2D) transition metal dichalcogenides (TMDCs) is an important step toward utilizing these materials for either electronics or catalysis. Here, we report a new surface-templated growth method that enables the fabrication of MoO

Chemically interfacing the inert basal plane of graphene with other materials has limited the development of graphene-based catalysts, composite materials, and devices. Here, we overcome this limitation by chemically activating epitaxial graphene on SiC(0001) using atomic oxygen. Atomic oxygen produces epoxide groups on graphene, which act as reactive nucleation sites for zinc oxide nanoparticle growth using the atomic layer deposition precursor diethyl zinc. In particular, exposure of epoxidized graphene to diethyl zinc abstracts oxygen, creating mobile species which diffuse on the surface to form metal oxide clusters. This mechanism is corroborated with a combination of scanning probe microscopy, Raman spectroscopy, and density functional theory, and can likely be generalized to a wide variety of related surface reactions on graphene.

Arc discharge single-walled carbon nanotubes (SWCNTs) possess superlative optical and electronic properties that are of high interest for technologically important applications including fiber optic communications, biomedical imaging, and field-effect transistors. However, as-grown arc discharge SWCNTs possess a mixture of metallic and semiconducting species in addition to a wide diameter distribution (1.2 to 1.7 nm) that limit their performance in devices. While previous postsynthetic sorting efforts have achieved separation by electronic type and diameter refinement for metallic arc discharge SWCNTs, tight diameter distributions of semiconducting arc discharge SWCNTs have not yet been realized. Herein, we present two advances in density gradient ultracentrifugation that enable the isolation of high purity (>99%) semiconducting arc discharge SWCNTs with narrow diameter distributions centered at ∼1.6 and ∼1.4 nm. The resulting diameter-refined populations of semiconducting arc discharge SWCNTs possess mon...

Electron solvation is examined at the interface of a room temperature ionic liquid (RTIL) and an Ag(111) electrode. Femtosecond two-photon photoemission spectroscopy is used to inject an electron into an ultrathin film of RTIL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyr](+)[NTf2](-)). While much of current literature highlights slower nanosecond solvation mechanisms in bulk ionic liquids, we observe only a femtosecond response, supporting morphology dependent and interface specific electron solvation mechanisms. The injected excess electron is found to reside in an electron affinity level residing near the metal surface. Population of this state decays back to the metal with a time constant of 400 ± 150 fs. Electron solvation is measured as a dynamic decrease in the energy with a time constant of 350 ± 150 fs. We observe two distinct temperature regimes, with a critical temperature near 250 K. The low temperature regime is characterized by a higher work function of 4.41 eV, while the high temperature regime is characterized by a lower work function of 4.19 eV. The total reorganizational energy of solvation changes above and below the critical temperature. In the high temperature regime, the electron affinity level solvates by 540 meV at 350 K, and below the critical temperature, solvation decreases to 200 meV at 130 K. This study will provide valuable insight to interface specific solvation of room temperature ionic liquids.

Molecular-scale control over the integration of disparate materials on graphene is a critical step in the development of graphene-based electronics and sensors. Here, we report that self-assembled monolayers of 10,12-pentacosadiynoic acid (PCDA) on epitaxial graphene can be used to template the reaction and directed growth of atomic layer deposited (ALD) oxide nanostructures with sub-10 nm lateral resolution. PCDA spontaneously assembles into well-ordered domains consisting of one-dimensional molecular chains that coat the entire graphene surface in a manner consistent with the symmetry of the underlying graphene lattice. Subsequently, zinc oxide and alumina ALD precursors are shown to preferentially react with the functional moieties of PCDA, resulting in templated oxide nanostructures. The retention of the original one-dimensional molecular ordering following ALD is dependent on the chemical reaction pathway and the stability of the monolayer, which can be enhanced via ultraviolet-induced molecular cross-linking.

The full potential of graphene in integrated circuits can only be realized with a reliable ultra-thin high-{\kappa} top-gate dielectric. Here, we report the first statistical analysis of the breakdown characteristics of dielectrics on graphene, which allows the simultaneous optimization of gate capacitance and the key parameters that describe large-area uniformity and dielectric strength. In particular, vertically heterogeneous and laterally homogenous Al2O3 and HfO2 stacks grown via atomic-layer deposition and seeded by a molecularly thin perylene-3,4,9,10-tetracarboxylic dianhydride organic monolayer exhibit high uniformities (Weibull shape parameter {\beta} > 25) and large breakdown strengths (Weibull scale parameter, EBD > 7 MV/cm) that are comparable to control dielectrics grown on Si substrates.

The fabrication of in-plane 2H-1T' MoTe

Organic photovoltaic (OPV) materials have recently garnered significant attention as enablers of high power conversion efficiency (PCE), low-cost, mechanically flexible solar cells. Nevertheless, further understanding-based materials developments will be required to achieve full commercial viability. In particular, the performance and durability of many current generation OPVs are limited by poorly understood interfacial phenomena. Careful analysis of typical OPV architectures reveals that the standard electron-blocking layer, poly-3,4-ethylenedioxy-thiophene:poly(styrene sulfonate) (PEDOT:PSS), is likely a major factor limiting the device durability and possibly performance. Here we report that a single layer of electronically tuned graphene oxide is an effective replacement for PEDOT:PSS and that it significantly enhances device durability while concurrently templating a performance-optimal active layer π-stacked face-on microstructure. Such OPVs based on graphene oxide exhibit PCEs as high as 7.5% whil...

Two of the primary hurdles facing organic electronics and photovoltaics are their low charge mobility and the inability to disentangle morphological and molecular effects on charge transport. Specific chemical groups such as alkyl side chains are often added to enable spin-casting and to improve overall power efficiency and morphologies, but their exact influence on mobility is poorly understood. Here, we use two-photon photoemission spectroscopy to study the charge transport properties of two organic semiconductors, one with and one without alkyl substituents (sexithiophene and dihexyl-sexithiophene). We show that the hydrocarbon side chains are responsible for charge localization within 230 fs. This implies that other chemical groups should be used instead of alkyl ligands to achieve the highest performance in organic photovoltaics and electronics.

We demonstrate the application of molecular rotational spectroscopy to measure the conformation isomerization rate of vibrationally excited pent-1-en-4-yne (pentenyne). The rotational spectra of single quantum states of pentenyne are acquired by using a combination of IR–Fourier transform microwave double-resonance spectroscopy and high-resolution, single-photon IR spectroscopy. The quantum states probed in these experiments have energy eigenvalues of ≈3,330 cm −1 and lie above the barrier to conformational isomerization. At this energy, the presence of intramolecular vibrational energy redistribution (IVR) is indicated through the extensive local perturbations found in the high-resolution rotation–vibration spectrum of the acetylenic C–H stretch normal-mode fundamental. The fact that the IVR process produces isomerization is deduced through a qualitatively different appearance of the excited-state rotational spectra compared with the pure rotational spectra of pentenyne. The rotational spectra of the vibrationally excited molecular eigenstates display coalescence between the characteristic rotational frequencies of the stable cis and skew conformations of the molecule. This coalescence is observed for quantum states prepared from laser excitation originating in the ground vibrational state of either of the two stable conformers. Experimental isomerization rates are extracted by using a three-state Bloch model of the dynamic rotational spectra that includes the effects of chemical exchange between the stable conformations. The time scale for the conformational isomerization rate of pentenyne at total energy of 3,330 cm −1 is ≈25 ps and is 50 times slower than the microcanonical isomerization rate predicted by the statistical Rice–Ramsperger–Kassel–Marcus theory.

Morphology- and layer-dependent electronic structure and dynamics at the PTCDA/Ag(111) interface have been studied with angle-resolved two-photon photoemission. In Stranski-Krastanov growth modes, the exposed wetting layer inhibited the evolution of the vacuum level and valence band to bulk values. For layer-by-layer growth, we observed the transition of electron structure from monolayer to bulk values within eight monolayers. Effective masses and lifetimes of the conduction band and the n = 1 image potential state were measured to be larger for disordered layers. The effective mass was interpreted in the context of charge mobility measurements.

A description of Fourier transform microwave (FTMW) detected infrared (IR) spectroscopy is presented. A series of measurements demonstrates the versatility of using a narrowband high Q cavity FTMW spectrometer as a rotationally resolved detector for molecular-beam infrared spectroscopy. The IR-FTMW spectrometer performance is characterized by comparing the spectrum of the acetylenic C–H stretch fundamental of 1-butyne to that obtained using a high-resolution electric resonance optothermal spectrometer (EROS). Two different measurement schemes, corresponding to IR excitation before or after the microwave polarization pulse, are compared. The ability to measure rotationally resolved spectra in the region of the first overtone of the acetylenic C–H stretch is demonstrated through measurements on tertbutylacetylene and trimethylsilylacetylene. One major advantage of the current technique is that large frequency scan rates can be achieved. This feature is useful for molecules exhibiting fast intramolecular vibrational energy redistribution (IVR) as shown by measurements of the highly perturbed acetylenic C–H stretch fundamental region of cyclopropylacetylene. The second strength of the technique is that the high-sensitivity of FTMW spectroscopy makes it possible to obtain the infrared spectra of molecules in low abundance in the pulsed jet expansion. This capability is exploited to measure the infrared spectrum of the 13 C isotopomer of cyclopropylacetylene in natural abundance and to obtain the spectra of the trans and gauche conformational isomers of 4-fluorobut-1-yne. The technique is also well-suited to measuring the vibrational spectra of weakly bound complexes. The spectral simplification achieved by the double-resonance measurement has made it possible to identify weak perturbations in the acetylenic C–H stretch fundamental region of the H–C C–H–NH 3 complex. The acetylenic C–H stretch fundamental spectrum of propyne–ammonia has also been obtained using the technique.

The fabrication of in-plane 2H-1T′ MoTe2 homojunctions by the flux-controlled, phase-engineering of few-layer MoTe2 from Mo nanoislands is reported. The phase of few-layer MoTe2 is controlled by simply changing Te atomic flux controlled by the temperature of the reaction vessel. Few-layer 2H MoTe2 is formed with high Te flux, while few-layer 1T′ MoTe2 is obtained with low Te flux. With medium flux, few-layer in-plane 2H-1T′ MoTe2 homojunctions are synthesized. As-synthesized MoTe2 is characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. Kelvin probe force microscopy and Raman mapping confirm that in-plane 2H-1T′ MoTe2 homojunctions have abrupt interfaces between 2H and 1T′ MoTe2 domains, possessing a potential difference of about 100 mV. It is further shown that this method can be extended to create patterned metal–semiconductor junctions in MoTe2 in a two-step lithographic synthesis. The flux-controlled phase engineering method could be utilized for the large-scale controlled fabrication of 2D metal–semiconductor junctions for next-generation electronic and optoelectronic devices.

We report infrared studies of adsorbed atomic oxygen (epoxide functional groups) on graphene. Two different systems are used as a platform to explore these interactions, namely, epitaxial graphene/SiC(0001) functionalized with atomic oxygen (graphene epoxide, GE) and chemically reduced graphene oxide (RGO). In the case of the model GE system, IR reflectivity measurements show that epoxide groups distort the graphene π bands around the K-point, imparting a finite effective mass and contributing to a band gap. In the case of RGO, epoxide groups are found to be present following the reduction treatment by a combination of polarized IR reflectance and transmittance measurements. Similar to the GE system, a band gap in the RGO sample is observed as well.

Ultra-thin MoS2 has recently emerged as a promising two-dimensional semiconductor for electronic and optoelectronic applications. Here, we report high mobility (>60 cm2/Vs at room temperature) field-effect transistors that employ unencapsulated single-layer MoS2 on oxidized Si wafers with a low level of extrinsic contamination. While charge transport in the sub-threshold regime is consistent with a variable range hopping model, monotonically decreasing field-effect mobility with increasing temperature suggests band-like transport in the linear regime. At temperatures below 100 K, temperature-independent mobility is limited by Coulomb scattering, whereas, at temperatures above 100 K, phonon-limited mobility decreases as a power law with increasing temperature.

Although graphene's physical structure is a single atom thick, two-dimensional, hexagonal crystal of sp(2) bonded carbon, this simple description belies the myriad interesting and complex physical properties attributed to this fascinating material. Because of its unusual electronic structure and superlative properties, graphene serves as a leading candidate for many next generation technologies including high frequency electronics, broadband photodetectors, biological and gas sensors, and transparent conductive coatings. Despite this promise, researchers could apply graphene more routinely in real-world technologies if they could chemically adjust graphene's electronic properties. For example, the covalent modification of graphene to create a band gap comparable to silicon (∼1 eV) would enable its use in digital electronics, and larger band gaps would provide new opportunities for graphene-based photonics. Toward this end, researchers have focused considerable effort on the chemical functionalization of graphene. Due to its high thermodynamic stability and chemical inertness, new methods and techniques are required to create covalent bonds without promoting undesirable side reactions or irreversible damage to the underlying carbon lattice. In this Account, we review and discuss recent theoretical and experimental work studying covalent modifications to graphene using gas phase atomic radicals. Atomic radicals have sufficient energy to overcome the kinetic and thermodynamic barriers associated with covalent reactions on the basal plane of graphene but lack the energy required to break the C-C sigma bonds that would destroy the carbon lattice. Furthermore, because they are atomic species, radicals substantially reduce the likelihood of unwanted side reactions that confound other covalent chemistries. Overall, these methods based on atomic radicals show promise for the homogeneous functionalization of graphene and the production of new classes of two-dimensional materials with fundamentally different electronic and physical properties. Specifically, we focus on recent studies of the addition of atomic hydrogen, fluorine, and oxygen to the basal plane of graphene. In each of these reactions, a high energy, activating step initiates the process, breaking the local π structure and distorting the surrounding lattice. Scanning tunneling microscopy experiments reveal that substrate mediated interactions often dominate when the initial binding event occurs. We then compare these substrate effects with the results of theoretical studies that typically assume a vacuum environment. As the surface coverage increases, clusters often form around the initial distortion, and the stoichiometric composition of the saturated end product depends strongly on both the substrate and reactant species. In addition to these chemical and structural observations, we review how covalent modification can extend the range of physical properties that are achievable in two-dimensional materials.

The superlative electronic properties of graphene suggest its use as the foundation of next generation integrated circuits. However, this application requires precise control of the interface between graphene and other materials, especially the metal oxides that are commonly used as gate dielectrics. Towards that end, organic seeding layers have been empirically shown to seed ultrathin dielectric growth on graphene via atomic layer deposition (ALD), although the underlying chemical mechanisms and structural details of the molecule/dielectric interface remain unknown. Here, confocal resonance Raman spectroscopy is employed to quantify the structure and chemistry of monolayers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on graphene before and after deposition of alumina with the ALD precursors trimethyl aluminum (TMA) and water. Photoluminescence measurements provide further insight into the details of the growth mechanism, including the transition between layer-by-layer growth and island formation. Overall, these results reveal that PTCDA is not consumed during ALD, thereby preserving a well-defined and passivating organic interface between graphene and deposited dielectric thin films.

With its exceptional charge mobility, graphene holds great promise for applications in next-generation electronics. In an effort to tailor its properties and interfacial characteristics, the chemical functionalization of graphene is being actively pursued. The oxidation of graphene via the Hummers method is most widely used in current studies, although the chemical inhomogeneity and irreversibility of the resulting graphene oxide compromises its use in high-performance devices. Here, we present an alternative approach for oxidizing epitaxial graphene using atomic oxygen in ultrahigh vacuum. Atomic-resolution characterization with scanning tunnelling microscopy is quantitatively compared to density functional theory, showing that ultrahigh-vacuum oxidization results in uniform epoxy functionalization. Furthermore, this oxidation is shown to be fully reversible at temperatures as low as 260 °C using scanning tunnelling microscopy and spectroscopic techniques. In this manner, ultrahigh-vacuum oxidation overcomes the limitations of Hummers-method graphene oxide, thus creating new opportunities for the study and application of chemically functionalized graphene.

Unoccupied electronic states are probed in epitaxial films of metal phthalocyanines on Ag(111) using angle-resolved two-photon photoemission spectroscopy. Quantum confinement of an image state wave function results in band folding and the opening of a band gap. Angle-resolved photoemission intensity measurements resolve the probability density of the wave function, and are able to identify the symmetry of each band. These results are compared with predictions from a two-dimensional scattering model, and they point to the importance of the spatial extent and symmetry of functional groups in predicting and controlling electronic structure on surfaces and in molecular thin films.

Ultrafast angle-resolved two photon photoemission was used to study the dynamics and interfacial band structure of ultrathin films adsorbed onto Ag(111). Studies focused on the image potential state (IPS) in each system as a probe for measuring changes in electronic behavior in differing environments. The energetics and dynamics of the IPS at the toluene/Ag(111) interface are strongly dependent upon coverage. For a single monolayer, the first IPS is bound by 0.81 eV below the vacuum level and has a lifetime of 50 femtoseconds (fs). Further adsorption of toluene creates islands of toluene with an exposed wetting layer underneath. The IPS is then split into two peaks, one corresponding to the islands and one corresponding to the monolayer. The wetting layer IPS shows the same dynamics as the monolayer, while the lifetime of the islands increases exponentially with increasing thickness. Furthermore, the island IPS transitions from delocalized to localized within 500 fs, and electrons with larger parallel momenta decay much faster. Attempts were made using a stochastic model to extract the rates of localization and intraband cooling at differing momenta. In sexithiophene (6T) and dihexyl-sexithiophene (DH6T), the IPS was used as a probe to see if the nuclear motion of spectatingmore » side chains can interfere with molecular conduction. The energy and band mass of the IPS was measured for 6T and two geometries of DH6T on Ag(111). Electrons injected into the thicker coverages of DH6T grew exponentially heavier until they were completely localized by 230 fs, while those injected into 6T remained nearly free electron like. Based off of lifetime arguments and the density of defects, the most likely cause for the mass enhancement of the IPS in this system is small polaron formation caused by coupling of the electron to vibrations of the alkyl substituents. The energetic relaxation of the molecular adsorbate was also measured to be 20 meV/100 fs for the DH6T, and 0 meV/100 fs for the 6T. This relaxation is consistent with the localization of the charge creating a barrier for it moving from one lattice site to a neighboring one. Finally, the IPS was used to study the evolution of the surface band gap at the Mg/Ag(111) interface. The Mg(0001) surface band gap lies 1.6 eV below the Fermi level, and consequently shows no peak in the projected density of states for the IPS. A method for creating layer by layer growth of Mg on Ag(111) was determined using Auger Spectroscopy and low energy electron diffraction. By monitoring the decay of the intensity of the IPS versus coverage, it was determined that four layers of magnesium on Ag(111) is sufficient to completely eliminate the surface band gap« less

Angle-resolved two-photon photoemission was used to study the energy relaxation, population decay, and localization dynamics of image potential state (IPS) electrons in ultrathin films of dimethylsulfoxide (DMSO) on an Ag(111) substrate. Dynamic energy shifts of 50 ± 10 meV and 220 ± 10 meV were observed for n = 1 IPS electrons at one monolayer and two monolayer coverages of DMSO, respectively. The difference in energy shifts is attributed to rotational hindrance of the molecular dipole in the chemisorptive first monolayer. The finding confirms the proposed mechanism for the low differential capacitance of dimethylsulfoxide at noble metal interfaces in solution. A novel description of the IPS as a surface capacitance is presented to facilitate comparisons with electrochemical systems.

We present the basic principles of dynamic rotational spectroscopy for the highly vibrationally excited symmetric top molecule trifluoropropyne (TFP,CF3CCH). Single molecular eigenstate rotational spectra of TFP were recorded in the region of the first overtone of the nu(1) acetylenic stretching mode at 6550 cm(-1) by infrared-pulsed microwave-Fourier transform microwave triple resonance spectroscopy. The average rotational constant (B) of the highly vibrationally mixed quantum states at 6550 cm(-1) is 2909.33 MHz, a value that is 40 MHz larger than the rotational constant expected for the unperturbed C-H stretch overtone (2869.39 MHz). The average rotational constant and rotational line shape of the molecular eigenstate rotational spectra are compared to the distribution of rotational constants expected for the ensemble of normal-mode vibrational states at 6550 cm(-1) that can interact by intramolecular vibrational energy redistribution (IVR). The normal-mode population distribution at 6550 cm(-1) can be described using a Boltzmann distribution with a microcanonical temperature of 1200 K. At this energy the rotational constant distribution in the normal-mode basis set is peaked at about 2910 MHz with a width of about 230 MHz. The distribution is slightly asymmetric with a tail to the high end. The experimentally measured dynamic rotational spectra are centered at the normal-mode distribution peak; however, the spectral width is significantly narrower (40 MHz) than normal-mode ensemble width (230 MHz). This reduction of the width, along with the Lorentzian shape of the eigenstate rotational spectra when compared to the Gaussian shape of the calculated ensemble distribution, illustrates the narrowing of the spectrum due to IVR exchange. The IVR exchange rate was determined to be 120 ps, about ten times faster than the rate at which energy is redistributed from the v=2 level of the acetylenic stretch.

The intercalation of various atomic species, such as hydrogen, to the interface between epitaxial graphene (EG) and its SiC substrate is known to significantly influence the electronic properties of the graphene overlayers. Here, we use high-resolution X-ray reflectivity to investigate the structural consequences of the hydrogen intercalation process used in the formation of quasi-free-standing (QFS) EG/SiC(0001). We confirm that the interfacial layer is converted to a layer structurally indistinguishable from that of the overlying graphene layers. This newly formed graphene layer becomes decoupled from the SiC substrate and, along with the other graphene layers within the film, is vertically displaced by ∼2.1 A. The number of total carbon layers is conserved during the process, and we observe no other structural changes such as interlayer intercalation or expansion of the graphene d-spacing. These results clarify the under-determined structure of hydrogen intercalated QFS-EG/SiC(0001) and provide a precise model to inform further fundamental and practical understanding of the system.

Angle- and time-resolved two-photon photoemission (TPPE) was used to investigate electronic states in the buffer layer of 4H-SiC(0001). An image potential state (IPS) series was observed on this strongly surface-bound buffer layer, and dispersion measurements indicated free-electron-like behavior for all states in this series. These results were compared with TPPE taken on bilayer graphene, which also show the existence of a free-electron-like IPS series. Lifetimes for the n = 2, and n = 3 states were obtained from time-resolved TPPE; slightly increased lifetimes were observed in the bilayer graphene sample for the n = 2 the n = 3 states. Despite the large band gap of graphene at the center of the Brillouin zone, the lifetime results demonstrate that the graphene layers do not behave as a simple tunneling barrier, suggesting that the buffer layer and graphene overlayers play a direct role in the decay of IPS electrons.