Highly Oriented Pyrolytic Graphite Samples Research Articles

Participation of Surface Oxygen in the Stabilization of the Rh/HOPG System with Respect to NO₂

In this work, using the method of X-ray photoelectron spectroscopy (XPS), a comparative study of the nature of the interaction of NO₂ at room temperature and a pressure of 10⁻⁵ mbar with two samples of highly oriented pyrolytic graphite (HOPG), on the surface of which rhodium was preliminarily deposited by vacuum deposition, was carried out. Before metal deposition, one of the HOPG samples was annealed in vacuum at 600°C, and the other was subjected to bombardment with argon ions, followed by exposure to air at room temperature for an hour in order to introduce strongly bound oxygen atoms into the surface composition. After deposition of rhodium on two samples of HOPG prepared, two model catalysts were obtained, designated as Rh/C and Rh/C(A)-O. It was found that the interaction of NO₂ with Rh/C led to the oxidation of graphite with the destruction of the surface layer. The Rh particles remained in the metallic state, but at the same time they were introduced into the near-surface layer of the carbon support. On the contrary, when the Rh/C(A)-O sample was treated with NO₂, the deposited rhodium was partially converted into RH₂O₃, while the graphite was oxidized to an insignificant degree and retained its original structure. The role of surface oxygen in the stabilization of graphite with respect to oxidation to NO₂ was discussed.

Work function measurement by ultraviolet photoelectron spectroscopy: Versailles project on advanced materials and standards interlaboratory study

We present the results of an interlaboratory study on work function (WF) measurements by ultraviolet photoelectron spectroscopy (UPS) conducted under the auspices of the Versailles Project on Advanced Materials and Standards. Two samples, gold (Au) film deposited on a flat Si(100) and highly oriented pyrolytic graphite (HOPG), were distributed to six different laboratories. Prior to UPS measurements, the samples underwent common sample pre-cleaning procedures: wet treatment and Ar+-sputtering in a vacuum for Au, and mechanical peeling in air for HOPG. Instrumental settings are adjusted for energy-scale calibration and sample bias optimization. The average reference WF value (WFref) and its expanded uncertainty (Uref) were calculated from each dataset that participants provided. The results of the Au sample demonstrated 5.40 ± 0.13 eV after Ar+ sputtering in vacuum, while the HOPG sample showed 4.62 ± 0.16 eV after mechanical peeling-off. Even though the participants used slightly different sample bias voltages and sample cleaning methods with various instruments, the results demonstrate remarkable consistency. Their consistency among the laboratories for both samples with high and low WF values provides a basis for establishing a new international standard for UPS in the surface chemical analysis community.

Application of ion implantation as a tool to study neutron induced morphological changes in HOPG and RBMK-1500 reactor graphite

Based on the RBMK-1500 (rus.: peaктop бoльшoй мoщнocти кaнaльный, PБMК; reaktor bolshoy moshchnosti kanalnyy, “high-power channel-type reactor”) reactor irradiation conditions, ion implantation was used as a tool to study neutron induced morphological changes in both highly oriented pyrolytic graphite (HOPG) and nuclear grade RBMK graphite. Graphite samples were implanted with 180 keV 14N+ ions at the fluences of 1.0 × 1016 ions/cm−2 and 2.5 × 1016 ions/cm−2. To study temperature effects on both ion migration and structural disorder level in the graphite matrix, the implantation procedures were carried out under different temperature conditions – room temperature (RT) and 500 °C. Subsequently, the distribution profiles of the implanted 14N+ ions were obtained by using secondary ion mass spectroscopy (SIMS) technique, while the microstructural properties of the graphite were evaluated by Raman spectroscopy. Implantation induced primary displacement damage profiles and implanted ion profiles were evaluated theoretically by using GEANT4 10.6 and SRIM-2013 codes. The profiles of implanted nitrogen obtained by SIMS technique were found to be in good agreement with theoretical ones. The surfaces of HOPG samples displayed macroscopic damage in form of fractures after 14N+ ion implantation at 500 °C; however, in case of RBMK graphite, the sample surfaces remained without visually observable changes. The Raman spectra showed an increase of sp3-related content and formation of amorphous carbon in both HOPG and RBMK graphite samples; the latter exhibits higher structural disorder at microscopic level and stronger amorphization than HOPG.

Chemisorption of CO2 on Nitrogen-Doped Graphitic Carbons.

The adsorption of CO2 on nitrogen-doped graphitic carbon materials, such as graphene nanosheet (GNS) powder and highly oriented pyrolytic graphite (HOPG), was comparatively studied using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Desorption of CO2 was observed at approximately 380 K for both pyridinic-nitrogen (pyri-N)-doped GNS and pyri-N-doped HOPG samples in the TPD experiments, whereas no CO2 desorption was observed for graphitic nitrogen-doped HOPG. This indicated that only pyri-N species create identical CO2 adsorption sites on any graphitic carbon surface. The adsorption energies of CO2 on pyri-N-doped carbons were estimated between 101 and 108 kJ mol-1, indicating that chemisorption, rather than physisorption, took place. The CO2 adsorption/desorption process was reproducible in repeated measurements, and no CO2 dissociation occurred during the process, suggesting that it is a promising CO2 capturing material. The O 1s peak of the adsorbed CO2 clearly appeared at 531.5-532 eV in the XPS measurements. The N 1s peak of pyri-N did not change with CO2 adsorption, indicating that CO2 is not directly bound to pyri-N but is adsorbed on a carbon atom near the pyridinic nitrogen via the nonbonding pz orbital of the carbon atom.

Unusual magnetic transition and enhanced diamagnetism in highly oriented pyrolytic graphite with long-range stacking-order

The recent observations of unusual superconductive effects in highly oriented pyrolytic graphite (HOPG) and twisted bi-layer, tri-layer and multilayer graphene materials have attracted an important attention. In this work we present a novel investigation on the relationship between structural ordering and magnetism in HOPG. Orientation dependent X-ray diffraction and temperature dependent magnetometry allowed for the identification of those HOPG samples exhibiting a long-range ordering along the c-axis. ZFC and FC magnetic curves acquired from T ~ 2 K to 300 K revealed a significant temperature-dependent variation of the diamagnetic component. By analyzing the ZFC/FC magnetic-curves (through magnetic moment subtraction methods) we identify the presence of a weak magnetic transition in the T range from 200 K to 2 K. The possible origin of the observed magnetization-trend is interpreted in relation to contributions arising from 1) disorder-rich locally-twisted-interfaces and 2) those internal graphitic sublattices exhibiting negative thermal expansion. Interestingly, magnetization against field measurements acquired at T ~ 50 K (max. field of 150 Oe) revealed the presence of a weak ferromagnetic-like hysteresis. Disappearance of the unusual ZFC/FC signal-splitting was instead found in those magnetic curves acquired from other HOPG samples with higher degree of disorder (short-range-ordering along the c-axis). This second category of samples was found to exhibit a disorder-mediated re-entrant spin-glass-like ferromagnetic behavior.

Superconducting-like and magnetic transitions in oxygen-implanted diamond-like and amorphous carbon films, and in highly oriented pyrolytic graphite

In our previously published work, we have reported colossal magnetoresistance, Andreev oscillations, ferromagnetism, and granular superconductivity in oxygen-implanted carbon fibers, graphite foils, and highly oriented pyrolytic graphite (HOPG). In this follow-up research, more results on these oxygen-implanted graphite samples are presented. We show results from transport measurements on oxygen-implanted diamond-like carbon thin coatings, amorphous carbon films, and HOPG. Significantly, a three-order magnitude drop in the electrical resistance of the oxygen-implanted diamond-like carbon films is observed at the 50 K temperature that we have previously reported for the transition to the superconducting state. Below 50 K, the films’ resistance oscillates between the high and low resistance states, less when the sample is under a transverse magnetic field. This metastability between the insulating and superconducting-like states possibly reflects the evolution of the amplitude for the superconducting order parameter also known as the longitudinal Higgs mode. Transitions to low resistance state and metastability are also observed for amorphous carbon films. Finally, the HOPG samples’ resistance have a thermally activated term that can be understood on the basis of the Langer–Ambegaokar–McCumber–Halperin model applied to narrow SC channels in which thermal fluctuations can cause phase slips. We also find that in oxygen-implanted carbon materials, the electron charge and spin correlations do not compete and their interplay rather facilitates the emergence of high-temperature superconductivity, and thus, additional unexpected effects like Heisenberg spin waves and magneto-structural transitions are observed.

In situ X-ray diffraction study on structural changes of neutron-irradiated highly oriented pyrolytic graphite under room-temperature compression and decompression

To clarify the nature of defects presented in neutron (n)-irradiated highly oriented pyrolytic graphite (HOPG), in situ X-ray diffraction (XRD) observation at room temperature (RT) and high pressure was conducted with synchrotron radiation (SPring-8). We focused on the graphite (002) [G(002)] peak under compression to 18.1 GPa and also under decompression. For comparison, unirradiated HOPG was also placed in the same high-pressure cell. We found that the G(002) peak can be represented by two components, the S and L peaks, for the n-irradiated HOPG, whereas it can be represented by only one component for the unirradiated HOPG. The d-spacing for the n-irradiated and unirradiated HOPG samples gradually decreased with increasing pressure. At 18.1 GPa, the d-spacing of the S peak of the irradiated sample became almost the same as that of the unirradiated one, but that of the L peak was larger. Under decompression, the behavior of the d-spacing was almost opposite to that under compression, and the d-spacing was restored to its value before compression. Also, taking account of the changes in the peak widths, we referred to and considered irradiation-induced defects of interstitial-type defects existing between the basal planes and in-plane defects of dislocation dipoles as possible defects that affect the changes in the G(002) peak.

Focused ion beam-based microfabrication of boron-doped diamond single-crystal tip cantilevers for electrical and mechanical scanning probe microscopy

In this paper, the fabrication process and electromechanical properties of novel atomic force microscopy probes utilising single-crystal boron-doped diamond are presented. The developed probes integrate scanning tips made of chemical vapour deposition-grown, freestanding diamond foil. The fabrication procedure was performed using nanomanipulation techniques combined with scanning electron microscopy and focused ion beam technologies. The mechanical properties of the cantilever were monitored by the measurement of thermally induced vibration of the cantilever after every fabrication step, allowing the mass changes in range of ng to be estimated. The endurance of the developed probes was tested during hundreds of topography measurements, which corresponds to a scanning length equal to 13.6 m, performed on a test sample in contact and lateral force microscopy modes. Analysis of the roughness parameters confirmed the extremely high wear resistance of the fabricated probes. The linear current voltage response on a highly-oriented pyrolytic graphite sample was recorded.

Interplay of edge/screw dislocations and enhanced magnetism in exfoliated pyrolytic graphite with distorted hexagonal moiré superlattices

The recent observation of enhanced magnetic signals in post-exfoliated lamellae of highly oriented pyrolytic graphite (HOPG) has attracted an important attention. It has been suggested that such a magnetization enhancement may originate from exfoliation-induced structural transitions, possibly involving rhombohedral domain and/or topological disorder formation. Further studies are therefore needed in order to elucidate the origin of such magnetic effects. Here we report a novel investigation on the structural and magnetic properties of multiple lamellae exfoliated from the inner regions of a main HOPG sample. HRTEM revealed moiré-superlattices with coexisting-periodicity-gradients ∇D∼13.1 nm to 16.2 nm, D∼ 2.6 nm in lamella-1, ∇D∼ 30 nm to 36 nm in lamella-2 and ∇D∼ 10.4 nm to ∼ 12.0nm, 3.1 nm in lamella-3. Exfoliation-induced distortion effects of the moiré-areas were investigated by employing HRTEM and Fourier transform analyses. We describe this effect on the basis of the Burgers–Bragg–Read–Shockley model with contributions arising from both I) edge dislocations (tilting angle θc ≠ 0, altered by the tape-exfoliation) and II) screw dislocations (twisting angle θtwist ≠ 0). Extended temperature dependent electron paramagnetic resonance (T-EPR) investigations revealed important shifts in the position of the π-electron differential absorption feature. The reported results are possibly an indicator of defect-induced electron correlation effects. Superconducting quantum interference device (SQUID) magnetometry measurements revealed an unusual magnetization-enhancement interpreted as a consequence of the exfoliation process.

Enhanced percolative spin-glass-like behavior in oxygen-rich highly oriented pyrolytic graphite with anomalous non-hexagonal superlattices and doubled electron diffraction patterns

The recent observations of superconductive-signals in grafoil and highly-oriented-pyrolytic-graphite (HOPG) have evidenced existence of unknown-defects responsible of electron-correlation-phenomena. Existence of anomalous-super-periodicities and doubled-diffraction-patterns has been also reported. Here we present an in-depth investigation of the structural-magnetic relationship of HOPG samples with unusual oxygen rich characteristics. By employing high-resolution-transmission-electron-microscopy (HRTEM) and selective-area-electron-diffraction (SAED) we demonstrate the appearance of non-hexagonal and hexagonal Moiré super-periodicities, with anomalous doubled electron diffraction patterns, in μm-thin lamellae exfoliated from HOPG. The hexagonal Moiré superlattices (D ~ 5.0 nm corresponding to θrot ~ 2.8o) highlight the presence of rotational misorientations between the graphitic layers and complex interfaces. Further, graphene oxide phases in direct interfacial contact with the graphitic layers were identified. Raman-spectroscopy evidenced D, D’ and G-bands compatible with the presence of sp3- and vacancy-rich configurations in proximity of the HOPG edges. Superconducting-quantum-interference-device (SQUID) revealed an enhanced spin-glass-like ferromagnetic behaviour. This observation was further confirmed by additional magnetometry measurements performed on graphite oxide. A diamagnetic background was found in all the analysed samples, while no superconductive signals were detected.

Streak Patterns Observed in Small Angle X-Ray Scattering from Highly Oriented Pyrolytic Graphite (HOPG)

Abstract Various forms of streak patterns are observed in small angle X-ray scattering from Highly Oriented Pyrolytic Graphite (HOPG). The streak patterns are assigned as specular reflection, double Bragg scattering, and diffuse + Bragg double scattering. These scatterings contain various information on the mesoscopic structures of the HOPG sample.

Anomalous T-induced second order diffraction loss and magnetic transition in red-wine-doped highly oriented pyrolytic graphite at the magic angle

Presence of high temperature superconductivity in highly-oriented-pyrolytic-graphite (HOPG) and water-doped-graphite has recently attracted important attention. In the latter, analogy with red-wine-doped FeTe1−x Sx superconductors has been proposed. We report a novel T-XRD investigation of red-wine-doped-HOPG at the magic angle (average θmisfit between the layers ∼0.8° as revealed by TEM and Moiré pattern analyses) from 77 K to 673 K. Anomalous T-induced depletion in the intensity of the 004 HOPG-reflection from 77 K to 348 K is demonstrated. Similar depletion features could be also found in comparative measurements performed in water doped HOPG samples. Significant deviation from the linear c-axis expansion predicted by the α-factor is shown together with a depletion in the graphitic c-axis shift-values for wine doped samples. EPR analyses are also provided and reveal a transition at 77 K implying possible presence of a change in the magnetic ordering (i.e. ferromagnetism) at low temperature. VSM analyses supported further the T-XRD results, by revealing presence of magnetic transition at approx. 350 K together with unusual superconductive-like hysteretic features at 250 K and 300 K which are not compatible with ferromagnetism.

Graphite to diamond transformation under shock compression: Role of orientational order

To gain insight into the role of orientational order on the shock-induced graphite to diamond phase transformation, three pyrolytic graphite types having different orientational orders were shock-compressed along the average c-axis to peak stresses between 35 and 69 GPa. The materials studied were ZYB-grade highly oriented pyrolytic graphite (HOPG), ZYH-grade HOPG, and as-deposited pyrolytic graphite (PG) having mosaic spreads of 0.8° ± 0.2°, 3.5° ± 1.5°, and ∼45°, respectively. Wave profiles, obtained using laser interferometry, show a multiple-wave structure with a distinct, rapid (<10 ns) rise to the high-pressure phase for each graphite type. Multiple-wave profiles, first observed in this study for the less ordered ZYH-grade HOPG and PG samples, show that somewhat poorly oriented pyrolytic graphites also undergo a well-defined phase transformation. Previously, rapid transformation was reported for ZYB-grade but not ZYH-grade HOPG. The measured wave profiles for both HOPG grades are very similar and both grades show a ∼22 GPa transformation stress. In contrast, the PG wave profiles are quite different and show a ∼46 GPa transformation stress. The continuum results (stress-density states) presented here cannot distinguish between the different high-pressure phases [hexagonal diamond (HD) or cubic diamond] reported in recent x-ray studies. Because ZYB-grade HOPG was recently shown to transform to HD and due to the similar peak states for both HOPG grades, it seems likely that ZYH-grade also transforms into HD. The very different shock responses of PG and HOPG suggest different transformation mechanisms for PG and HOPG, but the high-pressure PG phase remains unclear in the present work.

Inkjet printed graphene as an interconnect for optoelectronic devices

A comparative study of inkjet-printed graphene films (IPGFs) with mechanically exfoliated, highly crystalline graphene platelets have been conducted. Inkjet-printed graphene films were obtained using liquid-phase exfoliation of bulk graphite, while crystalline, residue-free graphene was obtained from highly-oriented-pyrolytic-graphite (HOPG) using mechanical exfoliation through a viscoelastic transfer process. Optical absorption spectroscopy was used to infer the density of platelets in the graphene-based ink dispersion. Temperature-dependent Raman spectroscopy revealed the presence of the defect D-band peak in the IPGFs, which was not observed in the HOPG-based samples at room temperature, confirming the higher crystalline quality of the latter. Full-width-half-maximum (FWHM) of the G-band was measured to be ~ 26.4 cm−1 for IPGFs compared to ~ 18.6 cm−1 for HOPG-based samples. Moreover, the D-band intensity decreased as temperature increased up to 600 °C for IPGFs, suggesting the possibility of annealing effects that may arise at these temperatures to reduce defect densities. In both HOPG-based samples and IPGF patterns, the G-band and G′-band red-shifted with increasing temperature which can be attributed to elongation of the C–C bond due to thermal expansion, resulting in the anharmonic coupling of the phonon modes. Moreover, a power study demonstrated the IPGFs even with printing passes as low as 10 passes, dissipate ~ 1.03 mW of power at 1 V, which was similar to the power dissipated in the HOPG samples (~ 1.05 mW at 1 V) suggesting good adherence of graphene platelets and high conductivity in IPGFs, which suggests that the inks are favorable for use in interconnects for device platforms in printed electronics. A natural follow-on from this work, was the use of the conductive graphene inks as an interconnect in devices, specifically WS2-based photodetectors, where prototype devices were fabricated and characterized that are also discussed here.

Intensity distribution profile of double Bragg scattering in the small-angle region from highly oriented pyrolytic graphite.

It is observed that radial streak patterns of double Bragg scattering appear in the small-angle X-ray scattering from highly oriented pyrolytic graphite (HOPG). The intensity profile of double Bragg scattering from an HOPG sample is calculated theoretically. Assuming that the c axes of the graphite crystallites in the HOPG sample are distributed around an orientation vector and their distribution function has a Gaussian form, it is found that the intensity profile of double Bragg scattering is expressed by a double Gaussian function of the scattering angle and the azimuthal angle of the streak. The calculated intensity profile is compared with the experimental one. The method developed in this article can be used to estimate the orientational distribution of crystallites in uniaxial polycrystalline materials.

Influence of interfaces on the transport properties of graphite revealed by nanometer thickness reduction

We investigated the influence of thickness reduction on the transport properties of graphite microflakes. Using oxygen plasma etching we decreased the thickness of highly oriented pyrolytic graphite (HOPG) microflakes from ∼100 nm to ∼20 nm systematically. Keeping current and voltage electrodes intact, the electrical resistance R(T), the magnetoresistance (MR) and Raman spectra were measured in every individual sample and after each etching step of a few nm. The results show that R(T) and MR can increase or decrease with the sample thickness in a non-systematic way. The results indicate that HOPG samples are inhomogeneous materials, in agreement with scanning transmission electron microscopy images and X-ray diffraction data. Our results further indicate that the quantum oscillations in the MR are not an intrinsic property of the ideal graphite structure but their origin is related to internal conducting interfaces.

Catalyst Design for Oxygen Reduction Reaction Using Pyridinic Nitrogen-Doped Carbon Materials

Nitrogen-doped graphitic carbons as non-Pt catalysts for oxygen reduction reaction (ORR) have been paid much attention in recent years because commercialization of fuel cells requires less expensive and abundant catalytic materials. Identification of the active site of nitrogen-doped carbon materials for ORR is thus urgently required, but still under debate. Currently, the debate focuses on whether the active site is created by pyridinic N (N bonded to two carbon atoms) or graphitic N (N bonded to three carbon atoms, also called substituted N or quaternary N). To determine the active site conclusively, we prepared model catalysts of highly oriented pyrolytic graphite (HOPG) with pyridinic N (pyri-HOPG) or graphitic N (grap-HOPG). The active sites and adsorption properties were examined by ORR and post-ORR X-ray photoelectron spectroscopy (XPS). We have thus determined the active nitrogen species in carbon [1].The graphitic-N doping was performed by mild bombardment with a nitrogen ion beam. To prepare the pyri-HOPG samples, edge patterning was first performed by bombarding the sample with an Ar+ ion beam through a slit-patterned Ni mask as shown in Fig.1 A-D. The edged HOPG samples were then exposed to NH3 at 973 K. The catalytic performances of the model catalysts were measured by cyclic voltammetry (CV) in acidic electrolyte (0.1 M H2SO4). It was found that the pyri-HOPG model catalyst shows high activity at high voltages, compared to the very low ORR activities of the N-free model catalysts (Fig.1 E, F). The pyri-HOPG sample with lower N concentration (N: 0.60 at%) is much more active than the grap-HOPG sample with higher N concentration (N: 0.73 at%). Since the pyri-HOPG sample is nearly free of graphitic N, the ORR results indicate that it is pyridinic N rather than the graphitic N that reduces the ORR overpotential and creates the active site. It is thus concluded that the ORR active sites in nitrogen-doped carbon materials are created by pyridinic N. Based on the model catalyst study, we then try to prepare catalytically active carbon surfaces covered with pyridinic nitrogen-containing aromatic molecules. That is, bottom-up preparation of catalysts using HOPG electrode covered with pyridinic nitrogen-containing aromatic molecules (dibenz[a,c] acridine (DA) molecule). The DA molecules were deposited on HOPG with different coverage by simply dropping solutions of the DA molecules at room temperature. Scanning tunneling microscopy (STM) measurements revealed that a well-ordered two-dimensional structure of DA monolayer is formed on HOPG surfaces with high densities via π-π interaction, rather than aggregates to form three-dimensional clusters. The nitrogen concentration of the DA-covered HOPG surfaces was estimated to be 0.5~1.5 at.% by XPS. The DA-covered HOPG model catalysts reveled activities of ORR. The specific activity per pyridinic nitrogen atom was estimated to be 0.08 (e sec-1 pyriN-1) at 0.3 eV, which is comparable to that for pyridinic nitrogen incorporated graphene sheets (0.07 ~ 0.14 (e sec-1 pyriN-1))[1]. The current densities at 0.1, 0.2, and 0.3 V vs RHE were in proportional to the surface coverage of DA molecules, indicating that the ORR active site was created by DA molecule adsorbed on HOPG. The present studies clearly show that fixing nitrogen-containing aromatic molecules on graphitic carbon materials is one of the promising approaches to prepare active ORR carbon catalysts. Acknowledgement: This study is supported by JST PRESTO and NEDO project. REFERENCES [1] D. Guo, R. Shibuyaa, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Science, 351 (2016) 361. Figure 1

Quenchable compressed graphite synthesized from neutron-irradiated highly oriented pyrolytic graphite in high pressure treatment at 1500 °C

The super hard material of “compressed graphite” (CG) has been reported to be formed under compression of graphite at room temperature. However, it returns to graphite under decompression. Neutron-irradiated graphite, on the other hand, is a unique material for the synthesis of a new carbon phase, as reported by the formation of an amorphous diamond by shock compression. Here, we investigate the change of structure of highly oriented pyrolytic graphite (HOPG) irradiated with neutrons to a fluence of 1.4 × 1024 n/m2 under static pressure. The neutron-irradiated HOPG sample was compressed to 15 GPa at room temperature and then the temperature was increased up to 1500 °C. X-ray diffraction, high-resolution transmission electron microscopy on the recovered sample clearly showed the formation of a significant amount of quenchable-CG with ordinary graphite. Formation of hexagonal and cubic diamonds was also confirmed. The effect of irradiation-induced defects on the synthesis of quenchable-CG under high pressure and high temperature treatment was discussed.

Laser-Induced Functionalization of Organo/Carbon Interfaces for Selective Adsorption of Au Nanoparticles in Microsized Domains.

Laser microprocessing of highly oriented pyrolytic graphite (HOPG) in conjunction with chemical functionalization routines is used to fabricate functional microsized domains. Infrared and Auger electron spectroscopy, contact angle measurements, and electron microscopy are used for characterization of laser-fabricated structures. HOPG samples are coated with alkylsiloxane monolayers. Laser-induced bromination of coated HOPG samples in gaseous bromine is carried out using a microfocused laser beam at a wavelength of 514 nm and 1/e2 laser spot diameter of about 2 μm. Subsequent azidation and amination results in functional domains with sizes in the range of 1.2 to 40 μm and more. At low laser powers and irradiation times fully functionalized circular-shaped structures are formed. At high laser powers and irradiation times laser processing results in decomposition of the organic monolayer and substrate in the center of the structures yielding donut-shaped structures. After laser processing and chemical transformation Au nanoparticles are selectively adsorbed onto the functional domains. This provides an opportunity to build up functional nanoparticle microarrays on carbon-based materials, e.g., for applications in sensing and electrocatalysis.

High repetition pump-and-probe photoemission spectroscopy based on a compact fiber laser system.

The paper describes a time-resolved photoemission (TRPES) apparatus equipped with a Yb-doped fiber laser system delivering 1.2-eV pump and 5.9-eV probe pulses at the repetition rate of 95 MHz. Time and energy resolutions are 11.3 meV and ∼310 fs, respectively, the latter is estimated by performing TRPES on a highly oriented pyrolytic graphite (HOPG). The high repetition rate is suited for achieving high signal-to-noise ratio in TRPES spectra, thereby facilitating investigations of ultrafast electronic dynamics in the low pump fluence (p) region. TRPES of polycrystalline bismuth (Bi) at p as low as 30 nJ/mm2 is demonstrated. The laser source is compact and is docked to an existing TRPES apparatus based on a 250-kHz Ti:sapphire laser system. The 95-MHz system is less prone to space-charge broadening effects compared to the 250-kHz system, which we explicitly show in a systematic probe-power dependency of the Fermi cutoff of polycrystalline gold. We also describe that the TRPES response of an oriented Bi(111)/HOPG sample is useful for fine-tuning the spatial overlap of the pump and probe beams even when p is as low as 30 nJ/mm2.