Scanning Tunneling Microscopy Data Research Articles

Research Update: Spatially resolved mapping of electronic structure on atomic level by multivariate statistical analysis

Atomic level spatial variability of electronic structure in Fe-based superconductor FeTe0.55Se0.45 (Tc = 15 K) is explored using current-imaging tunneling-spectroscopy. Multivariate statistical analysis of the data differentiates regions of dissimilar electronic behavior that can be identified with the segregation of chalcogen atoms, as well as boundaries between terminations and near neighbor interactions. Subsequent clustering analysis allows identification of the spatial localization of these dissimilar regions. Similar statistical analysis of modeled calculated density of states of chemically inhomogeneous FeTe1−xSex structures further confirms that the two types of chalcogens, i.e., Te and Se, can be identified by their electronic signature and differentiated by their local chemical environment. This approach allows detailed chemical discrimination of the scanning tunneling microscopy data including separation of atomic identities, proximity, and local configuration effects and can be universally applicable to chemically and electronically inhomogeneous surfaces.

Critical assessment of the evidence for striped nanoparticles.

There is now a significant body of literature which reports that stripes form in the ligand shell of suitably functionalised Au nanoparticles. This stripe morphology has been proposed to strongly affect the physicochemical and biochemical properties of the particles. We critique the published evidence for striped nanoparticles in detail, with a particular focus on the interpretation of scanning tunnelling microscopy (STM) data (as this is the only technique which ostensibly provides direct evidence for the presence of stripes). Through a combination of an exhaustive re-analysis of the original data, in addition to new experimental measurements of a simple control sample comprising entirely unfunctionalised particles, we show that all of the STM evidence for striped nanoparticles published to date can instead be explained by a combination of well-known instrumental artefacts, or by issues with data acquisition/analysis protocols. We also critically re-examine the evidence for the presence of ligand stripes which has been claimed to have been found from transmission electron microscopy, nuclear magnetic resonance spectroscopy, small angle neutron scattering experiments, and computer simulations. Although these data can indeed be interpreted in terms of stripe formation, we show that the reported results can alternatively be explained as arising from a combination of instrumental artefacts and inadequate data analysis techniques.

Static and dynamic buckling of reconstructions at triple steps on Si(111) surfaces

Triple steps on Si(111) surfaces are popular building blocks for bottom-up nanostructure assembly, conferring size uniformity and precise positioning of growing nanostructures. In this work, we employ the Si(7 7 10) regular stepped surface as model system to study the triple steps by scanning tunneling microscopy (STM) and large-scale first-principles calculations. We find a surprising cohabitation of reconstruction elements at the step edge that either buckles statically or dynamically at room temperature. The driving force for the observed sequence of buckling patterns is traced back to Coulomb interactions involving charged adatoms and rest-atoms lying on a mini-terrace. These results reconcile the Si(111) triple step model with the experimental STM data.

Monolayer Iron Carbide Films on Au(111) as a Fischer–Tropsch Model Catalyst

Using scanning tunneling microscopy (STM), we characterize the atomic-scale details of ultrathin films of iron carbide (FexCy) on Au(111) synthesized as a potential model system for the active iron carbide phase in iron Fischer–Tropsch synthesis (FTS) catalysts. The experiments show that room-temperature exposure of Fe islands gas to C2H4 deposited on the clean Au(111) surface results in partly converted Fe/FexCy islands. Multistep flash-heating treatment of the partly converted Fe/FexCy islands at 523 and 773 K results in pure highly crystalline FexCy islands with in-plane nearest-neighbor distances of 0.315 ± 0.005 nm. On the basis of the atom-resolved STM data, we propose that C2H4 dissociates at Fe island edges, after which the carbon diffuses inward into the interstitial region between the Fe and the Au substrate to form an FexCy surface that may be a good starting point for the investigation of iron carbide surfaces present under FTS conditions.

Palladium nanoparticle formation on TiO₂(110) by thermal decomposition of palladium(II) hexafluoroacetylacetonate.

Palladium nanoparticles were synthesized by thermal decomposition of palladium(II) hexafluoroacetylacetonate (Pd(hfac)2), an atomic layer deposition (ALD) precursor, on a TiO2(110) surface. According to X-ray photoelectron spectroscopy (XPS), Pd(hfac)2 adsorbs on TiO2(110) dissociatively yielding Pd(hfac)(ads), hfac(ads), and adsorbed fragments of the hfac ligand at 300 K. A (2 × 1) surface overlayer was observed by scanning tunneling microscopy (STM), indicating that hfac adsorbs in a bidentate bridging fashion across two Ti 5-fold atoms and Pd(hfac) adsorbs between two bridging oxygen atoms on the surface. Annealing of the Pd(hfac)(ads) and hfac(ads) species at 525 K decomposed the adsorbed hfac ligands, leaving PdO-like species and/or Pd atoms or clusters. Above 575 K, the XPS Pd 3d peaks shift toward lower binding energies and Pd nanoparticles are observed by STM. These observations point to the sintering of Pd atoms and clusters to Pd nanoparticles. The average height of the Pd nanoparticles was 1.2 ± 0.6 nm at 575 K and increased to 1.7 ± 0.5 nm following annealing at 875 K. The Pd coverage was estimated from XPS and STM data to be 0.05 and 0.03 monolayers (ML), respectively, after the first adsorption/decomposition cycle. The amount of palladium deposited on the TiO2(110) surface increased linearly with the number of adsorption/decomposition cycles with a growth rate of 0.05 ML or 0.6 Å per cycle. We suggest that the removal of the hfac ligand and fragments eliminates the nucleation inhibition of Pd nanoparticles previously observed for the Pd(hfac)2 precursor on TiO2.

Search for the Structure of a Sulfur-Induced Reconstruction on Cu(111)

We have carried out an extensive DFT-based search for the structure of the (√43 × √43)R ± 7.5° reconstruction of S on Cu(111), which exhibits a honeycomb-type structure in scanning tunneling microscopy (STM). We apply two criteria in this search: The structure must have a reasonably low chemical potential, and it must provide a good match with STM data, both our own and the data published by Wahlström et al. Phys. Rev. B 1999, 60, 10699. The best model has 12 S adatoms and 9 Cu adatoms per unit cell. Local defects within the Cu9S12 framework, consisting of one missing or one extra Cu adatom per unit cell, would be difficult to detect with STM and would not be energetically costly. There is no obvious correlation between this model and the structure of bulk CuS. If the √43 reconstruction is viewed in terms of local building blocks, then CuS3 and CuS2 clusters, linked by shared S atoms, provides the best description.

Chiral induction with chiral conformational switches in the limit of low "sergeants to soldiers" ratio.

Molecular-level insights into chiral adsorption phenomena are highly relevant within the fields of asymmetric heterogeneous catalysis or chiral separation and may contribute to understand the origins of homochirality in nature. Here, we investigate chiral induction by the "sergeants and soldiers" mechanism for an oligo(phenylene ethynylene) based chiral conformational switch by coadsorbing it with an intrinsically chiral seed on Au(111). Through statistical analysis of scanning tunneling microscopy (STM) data, we demonstrate successful chiral induction with a very low concentration of seeding molecules down to 3%. The microscopic mechanism for the observed chiral induction is suggested to involve nucleation of the intrinsically chiral seeds, allowing for effective transfer and amplification of chirality to large numbers of soldier target molecules.

Charge density waves as the origin of dip-hump structures in the differential tunneling conductance of cuprates: The case of d-wave superconductivity

Quasiparticle differential current–voltage characteristics (CVCs) G(V) of non-symmetric tunnel junctions between d-wave superconductors with charge-density waves (CDWs) and normal metals were calculated. The dependences G(V) were shown to have a V-like form at small voltages V and low temperatures, and to be asymmetric at larger V owing to the presence of CDW peak in either of the V-branches. The spatial scatter of the dielectric (CDW) order parameter smears the CDW peak into a hump and induces a peak-dip-hump structure (PDHS) typical of CVCs observed for such junctions. At temperatures larger than the superconducting critical one, the PDHS evolves into a pseudogap depression. The results agree well with the scanning tunneling microscopy data for Bi2Sr2CaCu2O8+δ and YBa2Cu3O7−δ. The results differ substantially from those obtained earlier for CDW s-wave superconductors.

Formation and atomic structure of ordered Sr-induced nanostrips on Ge(100)

The deposition of alkaline earths onto Ge(100) surfaces leads to well-ordered arrays of narrow trenches and elongated plateaus that extend for thousands of angstroms. Using scanning tunneling microscopy (STM) in conjunction with density functional theory (DFT), the atomic scale details of these nanostructures are revealed and the driving force responsible for their formation is evaluated. The STM data reveal a dramatic contrast reversal when the polarity of the imaging bias is switched. An energetically favorable structure for the plateaus was found using DFT that can reproduce all of the observed features. This structure is based upon a double dimer vacancy model in which Sr atoms displace two Ge dimers from the surface. Interestingly, the ordered plateau-trench structure is unique to Ge(100) despite the structural and chemical similarities to the Si(100) surface.

Surface diffusion of Pb atoms on the Si(553)-Au surface in narrow quasi-one-dimensional channels

The one-dimensional diffusion of individual Pb atoms on the Si(553)-Au surface has been investigated by a combination of scanning tunneling microscopy (STM), spectroscopy (STS), and first-principles density functional theory. The obtained results unambiguously prove that the diffusion channels are limited to a narrow region between Au chains and step edges of the surface. Much wider channels observed in STM and STS data have electronic origin and result from an interaction of Pb with surface atoms. The length of the channels is determined by a distance between defects at step edges of the Si(553)-Au surface. The defects can act as potential barriers or potential wells for Pb atoms, depending on their origin.

CO tip functionalization inverts atomic force microscopy contrast via short-range electrostatic forces.

We investigate insulating Cu2N islands grown on Cu(100) by means of combined scanning tunneling microscopy and atomic force microscopy with two vastly different tips: a bare metal tip and a CO-terminated tip. We use scanning tunneling microscopy data as proposed by Choi, Ruggiero, and Gupta to unambiguously identify atomic positions. Atomic force microscopy images taken with the two different tips show an inverted contrast over Cu2N. The observed force contrast can be explained with an electrostatic model, where the two tips have dipole moments of opposite directions. This highlights the importance of short-range electrostatic forces in the formation of atomic contrast on polar surfaces in noncontact atomic force microscopy.

Walking-like diffusion of two-footed asymmetric aromatic adsorbates on Pt(111)

We present a combined density functional theory (DFT) and scanning tunneling microscopy (STM) investigation of the adsorption and diffusion of non-interacting 2,2,2-trifluoroacetophenone (TFAP) and (R)-(+)-1-(1-naphthyl)ethylamine ((R)-NEA) on the Pt(111) surface. The two adsorbates are prochiral and chiral, respectively, and their surface diffusion is of specific interest in relation to chirality transfer from chiral modifiers to prochiral reactants in Orito-type hydrogenation on metal catalysts. Using a van der Waals-DFT description, the two adsorbates are shown to bind strongly to the surface through the aromatic groups and their respective side groups with carbonyl and amine functions. Adsorption energies were calculated for twelve different (R)-NEA adsorption conformers. A naming convention is introduced to describe the different conformers, and the two most stable ones are labeled (R)-NEA-1 and (R)-NEA-2. Diffusion of both adsorbates is calculated to proceed in a “walking” manner, moving either the aromatic group or the substituent group. For TFAP a diffusion barrier of 0.66eV is calculated while 0.74–0.89eV barriers are found for the two adsorption conformers of (R)-NEA. In agreement with the computed results, the room temperature STM study shows rapid diffusion of TFAP and significantly slower diffusion of (R)-NEA. Whereas TFAP monomers are rarely imaged at room temperature, consistent with rapid diffusion, time-lapsed images of (R)-NEA show limited molecular motion in localized regions of the surface. Furthermore, the motion of (R)-NEA is strongly conformation dependent. (R)-NEA-1 displays a much greater propensity to undergo rotational motion. The STM data for (R)-NEA are compared with DFT calculated energy barriers for translation and rotation. The possible implications of the experimental and calculated data for a role for induced fit stereodirection by (R)-NEA/Pt(111) are commented on.

Combined Photoemission and Scanning Tunneling Microscopy Study of the Surface-Assisted Ullmann Coupling Reaction

The adsorption and reaction of 4,4″-dibromo-para-terphenyl (DBTP) and 1,3,5-tris(4-bromophenyl)benzene (TBB) on Cu(111) surface were studied with X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. In addition, complementary scanning tunneling microscopy (STM) data are presented. At submonolayer coverage, scission of C–Br bonds occurs between 170 and 240 K. The estimated activation energy for this process is considerably lower than the C–Br bond energy, indicating that bond scission is assisted by Cu atoms of the substrate. The remaining molecular backbones undergo linkage by C–Cu–C bonds to form organometallic oligomers. Annealing of these oligomers leads to the formation of C–C bonded covalent two-dimensional networks. Above monolayer coverage, complete C–Br cleavage requires higher temperature, confirming the role of the Cu surface in the reaction. The results provide insight into the C–Br bond scission as the initial s...

Temperature-dependent scanning tunneling spectroscopy on the Si(557)-Au surface

Room-temperature and low-temperature (77 K) scanning tunneling spectroscopy (STS) and voltage-dependent scanning tunneling microscopy (STM) data are used to study the local electronic properties of the quasi-one-dimensional Si(557)-Au surface in real space. A gapped local electron density of states near the Gamma-point is observed at different positions of the surface, i.e., at protrusions arising from Si adatoms and step-edge atoms. Within the gap region, two distinct peaks are observed on the chain of localized protrusions attributed to Si adatoms. The energy gap widens on both types of protrusions after cooling from room temperature to T = 77 K. The temperature dependence of the local electronic properties can therefore not be attributed to a Peierls transition occurring for the step edge only. We suggest that more attention should be paid to finite-size effects on the one-dimensional segments.

Thermal evolution of the submonolayer near-surface alloy of ZnPd on Pd(111)

We have performed a high-resolution synchrotron radiation photoelectron spectroscopy study of the initial growth stages of the ZnPd near-surface alloy on Pd(111), complemented by scanning tunnelling microscopy data. We show that the chemical environment for surfaces containing less than half of one monolayer of Zn is chemically distinct from subsequent layers. Surfaces where the deposition is performed at room temperature contain ZnPd islands surrounded by a substrate with dilute Zn substitutions. Annealing these surfaces drives the Zn towards the substrate top-layer, and favours the completion of the first 1 : 1 monolayer before the onset of growth in the next layer.

Design and properties of a cryogenic dip-stick scanning tunneling microscope with capacitive coarse approach control

We present the design, setup, and operation of a new dip-stick scanning tunneling microscope. Its special design allows measurements in the temperature range from 4.7K up to room temperature, where cryogenic vacuum conditions are maintained during the measurement. The system fits into every (4)He vessel with a bore of 50mm, e.g., a transport dewar or a magnet bath cryostat. The microscope is equipped with a cleaving mechanism for cleaving single crystals in the whole temperature range and under cryogenic vacuum conditions. For the tip approach, a capacitive automated coarse approach is implemented. We present test measurements on the charge density wave system 2H-NbSe2 and the superconductor LiFeAs which demonstrate scanning tunneling microscopy and spectroscopy data acquisition with high stability, high spatial resolution at variable temperatures and in high magnetic fields.

Scanning tunneling microscopy and small angle neutron scattering study of mixed monolayer protected gold nanoparticles in organic solvents

When a binary mixture of ligand molecules is used to coat gold nanoparticles, stripe-like domains can occur. These nanodomains confer nanoparticles unique structure-dependent properties. The domain structure has been characterized primarily using scanning tunneling microscopy (STM) in air and in vacuum. Here we show the first STM images of striped nanoparticles in a solvent, 1-phenyloctane. We achieve stable imaging conditions on dodecanethiol–hexanethiol (C12 : C6) 2 : 1 protected gold nanoparticles. These features are persistent across many images and retain their direction and overall morphology when recorded at different scan angles. We also perform small angle neutron scattering (SANS) on two hybrid C6 : C12 nanoparticle samples dissolved in chloroform. The hybrid nanoparticles have the same composition and size distribution as samples imaged with STM, but one of the two ligands (either C6 or C12) is deuterated. Low resolution models reconstructed ab initio by simultaneous fitting of the SANS data reveal striped patterns of C6 and C12 on the gold surface. We use image analysis to quantitatively compare STM and SANS data, achieving remarkable agreement. This is the first paper to compare evidence of the existence of stripe-like domains for particles in solution using two independent techniques, and we believe that a combination of STM and SANS could become a major approach to characterize mixed ligand nanomaterials in solution.

Reconstructions and phase transition of clean Ge(110)

The structure of the clean Ge(110) surface is characterized between room temperature and the Ge melting temperature using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) and microscopy (LEEM). Rapid cooling from high temperature (~800°C) to room temperature yielded a surface composed of the c(8×10) reconstruction, {17 15 1} facets, and a previously unreported (8×2) reconstruction. Heating from room temperature to above 430°C extinguishes some, but not all, high-order LEED spots, indicating the presence of ordering up to at least 650°C. LEED observations of the phase transition between the c(8×10) and a disordered phase differ from earlier work but are consistent with previously published STM data.

Writing with atoms: Oxygen adatoms on the MoO2/Mo(110) surface

Writing at the nanoscale using the desorption of oxygen adatoms from the oxygen-rich MoO2+x/Mo(110) surface is demonstrated by scanning tunnelling microscopy (STM). High-temperature oxidation of the Mo(110) surface results in a strained, bulk-like MoO2(010) ultra-thin film with an O-Mo-O trilayer structure. Due to the lattice mismatch between the Mo(110) and the MoO2(010), the latter consists of well-ordered molybdenum oxide nanorows separated by 2.5 nm. The MoO2(010)/Mo(110) structure is confirmed by STM data and density functional theory calculations. Further oxidation results in the oxygen-rich MoO2+x/Mo(110) surface, which exhibits perfectly aligned double rows of oxygen adatoms, imaged by STM as bright protrusions. These adatoms can be removed from the surface by scanning (or pulsing) at positive sample biases greater than 1.5 V. Tip movement along the surface can be used for controlled lithography (or writing) at the nanoscale, with a minimum feature size of just 3 nm. By moving the STM tip in a predetermined fashion, information can be written and read by applying specific biases between the surface and the tip. Open image in new window

Packing of Isophthalate Tetracarboxylic Acids on Au(111): Rows and Disordered Herringbone Structures

Scanning tunnelling microscopy (STM) has been used to investigate the formation of hydrogen-bonded structures of the isophthalate tetracarboxylic acids, biphenyl-3,3′,5,5′-tetracarboxylic acid (BPTC), terphenyl-3,3″,5,5″-tetracarboxylic acid (TPTC), and quarterphenyl-3,3‴,5,5‴-tetracarboxylic acid (QPTC), via deposition from solution onto Au(111). STM data reveal that ordered structures can be formed from an aqueous solution leading to the formation of rows for the shortest acid BPTC, while the longer molecules TPTC and QPTC adopt a herringbone-like structure with significant degrees of disorder. The influence of solvent and substrate on the molecular ordering is discussed, and density functional theory is used to identify molecular models for these new phases.