Local Probe Microscopy Research Articles

(Invited) Tuning Photocatalytic Activity with Energetic and Kinetic Asymmetry at Coating-Stabilized Particulate Semiconductors

Semiconductor photocatalysts coevolve reductive and oxidative reactions at nanoscale proximity. The local charge separation and local coevolution have unique advantages: i) photocatalysts can operate in neutral pH water or water vapor instead of strong acid or base; ii) most semiconductors form a self-passivating layer at corroded surfaces to prevent corrosion propagation; and iii) nanoparticulate photoabsorbers or nanocrystalline films have achieved near-unity quantum efficiency, promising scale-up solar-to-chemical conversion. Despite the near-unity quantum efficiency achieved for SrTiO3 particles, the Edisonian approach is applied to the vast majority of photocatalysts, thus limiting progress. Technologically important semiconductors of 1.1 – 2.3 eV bandgaps and properly adjusted band edges, such as GaP and Si, can be ball milled into powder, followed by chemical passivation and protective coating processes. Thus, we introduce the Hu-lab-invented charge-transport coatings to protect semiconductors, such as silicon and gallium indium phosphide. We tune their photocatalytic activity by varying and measuring the energetics and kinetics at local reactive sites, in the following case studies.First, we combine photoelectrochemical cells with local potential-sensing probe microscopy to elucidate photocatalytic processes. In particular, we focus on local energetics and kinetics of multiple concurrent redox processes, i.e., two-redox chemistry. Figure 1A illustrates such a three-terminal photo-electro-catalytic cell setup, where the photoelectrode materials are photocatalysts by themselves. We showcase the efficient charge-separation design by fabricating photocatalysts with particulate semiconductors (e.g., commercial CdS powders, GaInP2 epitaxial layers, and ball-milled GaP nanoparticles), TiO2 coatings, and Rh cocatalysts for the concurring reduction of water and oxidation of reversible redox mediators or sacrificial reagents.We discovered that nanoscale site energetics are mutually dependent on local charge-transfer kinetics.[1] Besides, all known visible-light active photocatalysts of 1.1 – 2.3 eV bandgap photo-oxidize or corrode in water.[2] We discovered multi-functional protective coatings that i) allow for both electrons and holes to transport at tunable energy levels;[3] ii) shift band edges favorably, [4]; iii) reduce charge trapping and recombination,[5] and iv) tune water-oxidation selectivity between O2 and hydrogen peroxide production. [6] We show that in the presence of quinone redox couples in a bicarbonate solution, CO is produced with a 1-atm CO2-free Ar-purged headspace where the only source of CO2 is the (bi)carbonate anions, whilst the quinone redox couples can systematically vary the band edge positions of GaInP photoabsorbers.As shown in Figure 1B, CO evolution rate for both Fe(CN)6 3-/4- and H2BQ/BQ are higher at pH 7 than pH 8.5. The local CO2 concentration at pH 8.5 limited CO production rates. This observation corroborates our hypothesis that photocatalytic redox reactions locally drive pH swing to release CO2 at the oxidative sites, whereas the nearby reductive sites reduce in-situ generated CO2 into CO2R products. We envision direct solar fuel production from natural resources such as sunlight, bicarbonates from the ocean, or moisture in the air in a durable particle reactor.[7] We also processed wood and leaves into pulps and performed photocatalytic H2 production from the CdS/TiO2/Rh/CoOx panel (Figure 1C) using bottom LED illumination, as the glucose solution became colored.In summary, we show the rational design of photocatalysts. These coating-stabilized semiconductor particles present a scalable pathway for making H2 while effectively activating water and biomass as the two most abundant oxidants. We address the timescale mismatch between light absorption and catalysis by effectively accumulating electrons and holes at the respective reductive and oxidative sites.[8] The study paves the way for utilizing biomass to obtain bioenergy with carbon capture and storage.

Bridging the Gap between Single Nanoparticle Imaging and Global Electrochemical Response by Correlative Microscopy Assisted By Machine Vision.

The nanostructuration of an electrochemical interface dictates its micro- and macroscopic behavior. It is generally highly complex and often evolves under operating conditions. Electrochemistry at these nanostructurations can be imaged both operando and/or ex situ at the single nanoobject or nanoparticle (NP) level by diverse optical, electron, and local probe microscopy techniques. However, they only probe a tiny random fraction of interfaces that are by essence highly heterogeneous. Given the above background, correlative multimicroscopy strategy coupled to electrochemistry in a droplet cell provides a unique solution to gain mechanistic insights in electrocatalysis. To do so, a general machine-vision methodology is depicted enabling the automated local identification of various physical and chemical descriptors of NPs (size, composition, activity) obtained from multiple complementary operando and ex situ microscopy imaging of the electrode. These multifarious microscopically probed descriptors for each and all individual NPs are used to reconstruct the global electrochemical response. Herein the methodology unveils the competing processes involved in the electrocatalysis of hydrogen evolution reaction at nickel based NPs, showing that Ni metal activity is comparable to that of platinum.

Experimental discovery of structure–property relationships in ferroelectric materials via active learning

Emergent functionalities of structural and topological defects in ferroelectric materials underpin an extremely broad spectrum of applications ranging from domain wall electronics to high dielectric and electromechanical responses. Many of these functionalities have been discovered and quantified via local scanning probe microscopy methods. However, the search has until now been based on either trial and error, or using auxiliary information such as the topography or domain wall structure to identify potential objects of interest on the basis of the intuition of operator or pre-existing hypotheses, with subsequent manual exploration. Here we report the development and implementation of a machine learning framework that actively discovers relationships between local domain structure and polarization-switching characteristics in ferroelectric materials encoded in the hysteresis loop. The hysteresis loops and their scalar descriptors such as nucleation bias, coercive bias and the hysteresis loop area (or more complex functionals of hysteresis loop shape) and corresponding uncertainties are used to guide the discovery of these relationships via automated piezoresponse force microscopy and spectroscopy experiments. As such, this approach combines the power of machine learning methods to learn the correlative relationships between high-dimensional data, as well as human-based physics insights encoded into the acquisition function. For ferroelectric materials, this automated workflow demonstrates that the discovery path and sampling points of on- and off-field hysteresis loops are largely different, indicating that on- and off-field hysteresis loops are dominated by different mechanisms. The proposed approach is universal and can be applied to a broad range of modern imaging and spectroscopy methods ranging from other scanning probe microscopy modalities to electron microscopy and chemical imaging.

Face dependent footprints of carpet-like graphene films grown on polycrystalline silicon carbide

Continuous epitaxial graphene (EG) films have been grown on polished surfaces of dense polycrystalline SiC ceramics by heating in a Spark Plasma Sintering furnace (SPS). The confining of the sample into a graphite die and the use of a high pulsed dc current that triggers the joule heating of the system play a key role in the graphene formation by Si sublimation. The polycrystalline nature of the sample has allowed the comparison, through different local probe microscopies and micro-Raman spectroscopy, between EG films simultaneously grown on different SiC faces at exactly the same conditions, providing valuable information on their thickness, quality and stresses. Notably, whereas a graphene bilayer grows over Si-face grains, multilayer graphene (∼10 layers) forms on C-faces. The observed nano-electronic and nano-mechanical heterogeneities of the surface, showing up as differences in charge state and strain release mechanisms, are found to arise from the dependence of the EG properties when developed on each grain orientation; the same applies in the case of the macro-mechanical scratch resistance. SPS demonstrates to be a reliable and flexible methodology to prepare continuous graphene films on SiC components with many possibilities for scaling-up at low cost.

Optical manipulation of single flux quanta.

Magnetic field can penetrate into type II superconductors in the form of Abrikosov vortices, which are magnetic flux tubes surrounded by circulating supercurrents often trapped at defects referred to as pinning sites. Although the average properties of the vortex matter in superconductors can be tuned with magnetic fields, temperature or electric currents, handling of individual Abrikosov vortices remains challenging and has been demonstrated only with sophisticated scanning local probe microscopies. Here we introduce a far-field optical method based on local heating of the superconductor with a focused laser beam to realize a fast and precise manipulation of individual vortices, in the same way as with optical tweezers. This simple approach provides the perfect basis for sculpting the magnetic flux profile in superconducting devices like a vortex lens or a vortex cleaner, without resorting to static pinning or ratchet effects.

Atomic force spectroscopy and density-functional study of graphene corrugation on Ru(0001)

Graphene, a thinnest material in the world, can form moire structures on different substrates, including graphite, h-BN, or metal surfaces. In such systems the structure of graphene, i. e. its corrugation, as well as its electronic and elastic properties are defined by the combination of the system geometry and local interaction strength at the interface. The corrugation in such structures on metals is heavily extracted from diffraction or local probe microscopy experiments and can be obtained only via comparison with theoretical data, which usually simulate the experimental findings. Here we show that graphene corrugation on metals can be measured directly employing atomic force spectroscopy and obtained value coincides with state-of-the-art theoretical results. We also address the elastic reaction of the formed graphene nanodoms on the indentation process by the scanning tip that is important for the modeling and fabrication of graphene-based nanoresonators on the nanoscale.

Imaging of Photonic Crystal Localized Modes through Third-Harmonic Generation

We demonstrate high-spatial resolution imaging of localized cavity modes through third-harmonic frequency conversion. The experiments are performed with a III-nitride-on-silicon photonic platform. The resonant cavities are formed within suspended two-dimensional photonic crystals and are excited with a continuous-wave excitation around 1550 nm. The III-nitride materials (GaN and AlN) are transparent for both pump and harmonics. The third-harmonic nonlinear process allows one to indirectly observe the fundamental confined cavity mode with spatial profiles equivalent to those usually obtained by local probe microscopy techniques such as scanning near-field optical microscopy. An excellent agreement is obtained between the measured polarization-resolved third-harmonic emission patterns and those calculated through the third-order nonlinear polarization. We show that the spatial profiles of the radiated patterns are strongly dependent on defocusing, thus highlighting the strong sensitivity of the imaging.

How Biphthalocyanine Molecules Do Arrange on Single Crystalline Surfaces

Phthalocyanine molecules have been the subject of a large number of investigations due to their versatile applications in the area of gas-sensing devices, photovoltaicmaterials, light-emitting diodes, solar and fuel cells and molecular magnets. There is an evident need to characterize the molecular arrangement and the electronic properties of such organic molecules on the substrates, for getting a better understanding of the molecule-molecule as well as molecule-surface interactions.To achieve this goal, it is crucial to characterize the molecular arrangement of the molecules and link their physical properties to the structure of the film. In this study, we focus on the double decker biphthalocyanine of lutetium (LuPc2), which interest relies in its particular electronic properties. We have studied the growth and structure of LuPc2 molecules on clean and passivated Si(100)-2x1 and also on Au(111) surface, by molecular vapor deposition under vacuum [1, 2, 3].We have used various surface experimental techniques such as surface differential optical spectroscopies, local probe microscopies and electron spectroscopies. We identified the growth mode and found that on the passivated silicon surface favors the self-organization of the molecules and we evidenced a strong influence of the flux during the formation of the molecular films. We could then propose the scenario for the growth of LuPc2 molecules on the two silicon surfaces, clean and passivated ones from few molecules up to about 40 nm.On Au(111) surface, molecules are found to absorb flat on the surface forming a bilayer [5] at initial coverages whereas clusters develop for larger coverages influencing their electronic structure.

Voltage induced local hysteretic phase switching in silicon

We report the observation of dc-bias induced 180° phase switching in silicon wafers by local-probe microscopy and spectroscopy. The switching is hysteretic and shows remarkable similarities with polarization switching in ferroelectrics as seen in piezoresponse force microscopy (PFM). This is always accompanied by a hysteretic amplitude vs. voltage curve which resembles the “butterfly loops” for piezoelectric materials. From a detailed analysis of the data obtained under different environmental and experimental conditions, we show that the hysteresis effects in phase and amplitude do not originate from ferro-electricity or piezoelectricity. This further indicates that mere observation of hysteresis effects in PFM does not confirm the existence of ferroelectric and/or piezoelectric ordering in materials. We also show that when samples are mounted on silicon for PFM measurements, the switching properties of silicon may appear on the sample even if the sample thickness is large.

Spatial resolution of confocal XRF technique using capillary optics

XRF (X-ray fluorescence) is a powerful technique for elemental analysis with a high sensitivity. The resolution is presently limited by the size of the primary excitation X-ray beam. A test-bed for confocal-type XRF has been developed to estimate the ultimate lateral resolution which could be reached in chemical mapping using this technique. A polycapillary lens is used to tightly focus the primary X-ray beam of a low power rhodium X-ray source, while the fluorescence signal is collected by a SDD detector through a cylindrical monocapillary. This system was used to characterize the geometry of the fluorescent zone. Capillary radii ranging from 50 μm down to 5 μm were used to investigate the fluorescence signal maximum level This study allows to estimate the ultimate resolution which could be reached in-lab or on a synchrotron beamline. A new tool combining local XRF and scanning probe microscopy is finally proposed.

Resolving oxide surfaces – From point and line defects to complex network structures

AbstractIn the following, we demonstrate the atomic‐scale analysis of oxide surfaces. Essential physical properties were extracted using noncontact atomic force microscopy (nc‐AFM) and scanning tunneling microscopy (STM). The main focus has been put on the determination of surface structures. A review of the recent achievements towards atomic‐scale resolution from highly crystalline to amorphous materials is provided. An overview of local probe microscopy and spectroscopy to get beyond the averaging character of diffraction methods is thereby summarized. In particular, surface defects of various dimensionality were investigated. Furthermore, acquisition of information on electronic properties is detailed. The presented material covers zero‐dimensional (0D) point defects, one‐dimensional (1D) line defects, and two‐dimensional (2D) random networks, i.e., amorphous structures. First, we present spectroscopy data taken on thin MgO films grown on Ag(001). Distance‐ and bias‐dependent nc‐AFM and STM measurements were recorded on these films. The local work‐function shift and electronic structure of color centers in the MgO surface were studied. In the next section, the structure determination of ultrathin alumina/NiAl(110) is shown. Atomically resolved nc‐AFM reveals a detailed picture of various line defects in the film. Finally, we discuss the atomic structure of a recently discovered ultrathin vitreous silica film on Ru(0001). The atomic arrangement in the 2D random network, resembling the classical picture of Zachariasen, is analyzed in terms of the pair correlation function and ring‐size distribution.

Growth mode and self-organization of LuPc2on Si(001)-2×1vicinal surfaces: An optical investigation

We report an investigation of the initial growth and of the self-organization of lutetium biphthalocyanine LuPc2 on Si(001)-2 x 1 vicinal surfaces. Using surface-sensitive optical spectroscopies, namely, surface-difference-reflectance spectroscopy (SDRS) and reflectance-anisotropy spectroscopy (RAS), together with local-probe microscopies, we are able to propose a scenario for the growth mode up to about 20 nm. We demonstrate that the growth mode initially proceeds through the formation of a wetting layer, followed by the formation of clusters whose sizes increase while keeping a constant shape in which the molecules are inclined. Moreover, the LuPc2 molecules are self-organized along the step edges, and we are able to estimate that about 30% to 100% of the molecules are aligned when considering that the molecules are tilted by 45 degrees to 63 degrees with respect to the surface normal.

Multiaxis interferometric displacement measurement for local probe microscopy

Abstract We present an overview of design approaches for nanometrology measuring setups with a focus on interferometry techniques and associated problems. The design and development of a positioning system with interferometric multiaxis monitoring and control is presented. The system is intended to operate as a national nanometrology standard combining local probe microscopy techniques and sample position control with traceability to the primary standard of length.

Ferroelectric Materials: Conduction at Domain Walls in Insulating Pb(Zr0.2Ti0.8)O3 Thin Films (Adv. Mater. 45/2011)

Electrical conduction at domain walls in the widely used ferroelectric insulator Pb(Zr0.2Ti0.8)O3 is demonstrated in local probe microscopy measurements by Patrycja Paruch and co-workers. The results suggest that this phenomenon, previously observed only in the complex multiferroic BiFeO3, is much more general than previously thought. Such nanoscale conduction channels offer great promise for potential applications in miniaturized devices.

Multidimensional interferometric tool for the local probe microscopy nanometrology

This work reports on the measurement at the nanoscale using local probe microscopy techniques, primarily atomic force microscopy. Recent applications using the atomic force microscope as a nanometrology tool require that not only the positioning of the tip has to be based on precise measurements but also the traceability of the measuring technique has to be ensured up to the primary standard. Thus, in our experimental work, laser interferometric measuring methods were employed. In this paper, a new design of the six-axis-dimensional interferometric measurement tool for local probe microscopy stage nanopositioning is presented.

Anodic bonded graphene

We show how to prepare graphene samples on a glass substrate with the anodic bonding method. In this method, a graphite precursor in flake form is bonded to a glass substrate with the help of an electrostatic field and then cleaved off to leave few layer graphene on the substrate. Now that several methods are available for producing graphene, the relevance of our method is in its simplicity and practicality for producing graphene samples of about 100 µm lateral dimensions. This method is also extensible to other layered materials. We discuss some detailed aspects of the fabrication and results from Raman spectroscopy, local probe microscopy and transport measurements on these samples.

Molecular monolayers on silicon as substrates for biosensors

(111) silicon surfaces can be controlled down to atomic level and offer a remarkable starting point for elaborating nanostructures. Hydrogenated surfaces are obtained by oxide dissolution in hydrofluoric acid or ammonium fluoride solution. Organic species are grafted onto the hydrogenated surface by a hydrosilylation reaction, providing a robust covalent Si–C bonding. Finally, probe molecules can be anchored to the organic end group, paving the way to the elaboration of sensors. Fluorescence detection is hampered by the high refractive index of silicon. However, improved sensitivity is obtained by replacing the bulk silicon substrate by a thin layer of amorphous silicon deposited on a reflector. The development of a novel hybrid SPR interface by the deposition of an amorphous silicon–carbon alloy is also presented. Such an interface allows the subsequent linking of stable organic monolayers through Si–C bonds for a plasmonic detection. On the other hand, the semiconducting properties of silicon can be used to implement field-effect label-free detection. However, the electrostatic interaction between adsorbed species may lead to a spreading of the adsorption isotherms, which should not be overlooked in practical operating conditions of the sensor. Atomically flat silicon surfaces may allow for measuring recognition interactions with local-probe microscopy.

Local probe microscopy with interferometric monitoring of the stage nanopositioning

We present a system of positioning and interferometric monitoring of a sample position for measurements and calibration in the nanoscale in metrology. The positioning is based on a three-axis stage which allows replacing scanning by the probe of an atomic force microscope with a system with full interferometric displacement measurement. A stage with 200 µm × 200 µm of horizontal travel extends also the microscope range. The stage allows positioning with sub-nanometer resolution in all three axes under a closed loop control with position detection via capacitive sensors. Interferometric system monitoring all six degrees of freedom of the stage ensures full metrological traceability of the positioning to the fundamental etalon of length and improves resolution and overall precision of the displacement monitoring.

Interferometric Displacement Measurement for Local Probe MicroscopyInterferometrische Abstandsmessung für das scannende Mikroskopieverfahren

Abstract In the contribution we present an interferometric displacement monitoring system for positioning of a sample stage for local probe microscopy. The arrangement is designed for metrology applications with direct link to fundamental etalon of length and monitoring of all six axes of freedom of motion. Active position of control is proposed.

Relaxation phenomena at the metal-to-insulator transition in La 0.8Sr 0.2MnO 3 single crystals

The time dependence of the resistance R A C of a La 0.8Sr 0.2MnO 3 single crystal has been investigated in the vicinity of the metal-to-insulator transition temperature. We used local probe microscopy to show the existence, at room temperature, of coexisting clusters of micrometer size. Our analysis shows that relaxation effects can be described with a simple exponential contribution using a random resistor-network, based on phase separation between insulating and metallic domains. Our results clearly prove the existence of a percolation threshold over which no percolation path exists. Moreover, these results highlight the significant role of the remanent magnetization.