Noncontact Atomic Force Microscopy Research Articles

Electronic Self-Passivation of Single Vacancy in Black Phosphorus via Ionization.

We report that monoelemental black phosphorus presents a new electronic self-passivation scheme of single vacancy (SV). By means of low-temperature scanning tunneling microscopy and noncontact atomic force microscopy, we demonstrate that the local reconstruction and ionization of SV into negatively charged SV^{-} leads to the passivation of dangling bonds and, thus, the quenching of in-gap states, which can be achieved by mild thermal annealing or STM tip manipulation. SV exhibits a strong and symmetric Friedel oscillation (FO) pattern, while SV^{-} shows an asymmetric FO pattern with local perturbation amplitude reduced by one order of magnitude and a faster decay rate. The enhanced passivation by forming SV^{-} can be attributed to its weak dipolelike perturbation, consistent with density-functional theory numerical calculations. Therefore, self-passivated SV^{-} is electrically benign and acts as a much weaker scattering center, which may hold the key to further enhance the charge mobility of black phosphorus and its analogs.

Resolution of Intramolecular Dipoles and a Push-Back Effect of Individual Molecules on a Metal Surface

Molecules consisting of a donor and an acceptor moiety can exhibit large intrinsic dipole moments. Upon deposition on a metal surface, the dipole may be effectively screened and the charge distribution altered due to hybridization with substrate electronic states. Here, we deposit ethyl-diaminodicyanoquinone molecules, which exhibit a large dipole moment in the gas phase, on a Au(111) surface. Employing a combination of scanning tunneling microscopy and noncontact atomic force microscopy, we find that a significant dipole moment persists in the flat-lying molecules. Density functional theory calculations reveal that the dipole moment is even increased on the metal substrate as compared to the gas phase. We also show that the local contact potential across the molecular islands is decreased by several tens of meV with respect to the bare metal. We explain this by the induced charge-density redistribution due to the adsorbed molecules, which confine the substrate’s wavefunction at the interface. Our local measurements provide direct evidence of this so-called push-back or cushion effect at the scale of individual molecules.

Substrate-Modulated Synthesis of Metal-Organic Hybrids by Tunable Multiple Aryl-Metal Bonds.

Assembly of semiconducting organic molecules with multiple aryl-metal covalent bonds into stable one- and two-dimensional (1D and 2D) metal-organic frameworks represents a promising route to the integration of single-molecule electronics in terms of structural robustness and charge transport efficiency. Although various metastable organometallic frameworks have been constructed by the extensive use of single aryl-metal bonds, it remains a great challenge to embed multiple aryl-metal bonds into these structures due to inadequate knowledge of harnessing such complex bonding motifs. Here, we demonstrate the substrate-modulated synthesis of 1D and 2D metal-organic hybrids (MOHs) with the organic building blocks (perylene) interlinked solely with multiple aryl-metal bonds via the stepwise thermal dehalogenation of 3,4,9,10-tetrabromo-1,6,7,12-tetrachloroperylene and subsequent metal-organic connection on metal surfaces. More importantly, the conversion from 1D to 2D MOHs is completely impeded on Au(111) but dominant on Ag(111). We comprehensively study the distinct reaction pathways on the two surfaces by visually tracking the structural evolution of the MOHs with high-resolution scanning tunneling and noncontact atomic force microscopy, supported by first-principles density functional theory calculations. The substrate-dependent structural control of the MOHs is attributed to the variation of the M-X (M = Au, Ag; X = C, Cl) bond strength regulated by the nature of the metal species.

Adsorption and self-assembly of a mono-cyano Zn-tetraphenylporphyrin derivative on KBr(001) and MgO(001)

The adsorption and self-assembly of a Zn-tetraphenylporphyrin derivative (Zn-pCNTPP) with a cyano group was investigated on KBr(001) and MgO(001) using low-temperature non-contact atomic force microscopy (nc-AFM) combined with dispersion-corrected density-functional theory (DFT). The deposition of Zn-pCNTPP at submonolayer coverage leads on both surfaces to self-assembled networks, in which the porphyrin assumes a planar adsorption configuration with the macrocycle parallel to the surface. DFT calculations confirm the planar adsorption geometry and reveal important differences in the adsorption on KBr vs. MgO. While a global minimum structure is found on KBr due to a strong CN⋯K interaction, multiple and energetically nearly equivalent adsorption sites occur on MgO. Therefore, commensurate adsorption on KBr is suggested, while optimizing the molecule–molecule over molecule–surface interactions is more important on MgO, which is experimentally both evidenced by the nc-AFM data. The expected Zn adsorption site ontop of the anion is the energetically most favored configuration only on KBr, while on MgO, it is above the surface cation.

On-Surface Debromination of C6Br6: C6 Ring versus C6 Chain

Carbon allotropes comprising sp-hybridized carbon atoms have been investigated for decades for their molecular structure. One of the unsolved mysteries is whether they should take a linear or cyclic configuration in condensed phases due to the lack of atomistic characterizations. Herein, we designed a molecule with a C6 skeleton as a model system to address this issue, which was achieved by eliminating Br atoms from hexabromobenzene (C6Br6) molecule on the Ag(111) substrate via thermal treatment. It is found that the C6 ring intermediate resulting from complete debromination is energetically unstable at room temperature based on theoretical calculations. It subsequently transforms into the C6 polyynic chain via a ring-opening process and ultimately polymerizes into the organometallic polyyne, whose triyne structural unit is revealed by bond-resolved noncontact atomic force microscopy. Theoretical calculations demonstrated an energetically favorable pathway in which the ring-opening process occurs after complete debromination of C6Br6. Our study provides a platform for the synthesis of elusive carbon-rich materials.

Development of a near-5-Kelvin, cryogen-free, pulse-tube refrigerator-based scanning probe microscope.

We report the design and performance of a cryogen-free, pulse-tube refrigerator (PTR)-based scanning probe microscopy (SPM) system capable of operating at a base temperature of near 5 K. We achieve this by combining a home-made interface design between the PTR cold head and the SPM head, with an automatic gas-handling system. The interface design isolates the PTR vibrations by a combination of polytetrafluoroethylene and stainless-steel bellows and by placing the SPM head on a passive vibration isolation table via two cold stages that are connected to thermal radiation shields using copper heat links. The gas-handling system regulates the helium heat-exchange gas pressures, facilitating both the cooldown to and maintenance of the base temperature. We discuss the effects of each component using measured vibration, current-noise, temperature, and pressure data. We demonstrate that our SPM system performance is comparable to known liquid-helium-based systems with the measurements of the superconducting gap spectrum of Pb, atomic-resolution scanning tunneling microscopy image and quasiparticle interference pattern of Au(111) surface, and non-contact atomic force microscopy image of NaCl(100) surface. Without the need for cryogen refills, the present SPM system enables uninterrupted low-temperature measurements.

On-surface polyarylene synthesis by cycloaromatization of isopropyl substituents

Immobilization of organic molecules on metal surfaces and their coupling via thermally induced C–C bond formation is an important technique in organic and polymer synthesis. Using this approach, insoluble and reactive carbon nanostructures can be synthesized and the reactions monitored in situ using scanning probe microscopy methods. The diversity of conceivable products, however, is limited by the number and variety of known on-surface reactions. Here, we introduce the on-surface synthesis of polyarylenes by intermolecular oxidative coupling of isopropyl substituents of arenes. This [3+3] dimerization reaction forms a new phenylene ring and can be regarded as a formal cycloaromatization. The synthetic value of this reaction is proved by the synthesis of polyarylenes and co-polyarylenes, which we demonstrate by synthesizing poly(2,7-pyrenylene-1,4-phenylene). Scanning tunnelling microscopy and non-contact atomic force microscopy studies, complemented by density functional theory calculations, offer mechanistic insight into the on-surface cycloaromatization reaction. On-surface methods can be used to synthesize organic molecules, polymers and nanomaterials, however, the diversity of conceivable products is limited by the number of known on-surface reactions. Now, a phenylene ring-forming reaction on a gold surface by intermolecular oxidative coupling of isopropyl substituents on arenes is reported. The reaction is probed using bond-resolved imaging and computational modelling.

Hidden surface photovoltages revealed by pump probe KPFM

In this work, we use pump-probe Kelvin probe force microscopy (pp-KPFM) in combination with non-contact atomic force microscopy (nc-AFM) under ultrahigh vacuum, to investigate the nature of the light-induced surface potential dynamics in alumina-passivated crystalline silicon, and in an organic bulk heterojunction thin film based on the PTB7-PC71BM tandem. In both cases, we demonstrate that it is possible to identify and separate the contributions of two different kinds of photo-induced charge distributions that give rise to potential shifts with opposite polarities, each characterized by different dynamics. The data acquired on the passivated crystalline silicon are shown to be fully consistent with the band-bending at the silicon-oxide interface, and with electron trapping processes in acceptors states and in the passivation layer. The full sequence of events that follow the electron–hole generation can be observed on the pp-KPFM curves, i.e. the carriers spatial separation and hole accumulation in the space charge area, the electron trapping, the electron–hole recombination, and finally the electron trap-release. Two dimensional dynamical maps of the organic blend photo-response are obtained by recording the pump-probe KPFM curves in data cube mode, and by implementing a specific batch processing protocol. Sample areas displaying an extra positive SPV component characterized by decay time-constants of a few tens of microseconds are thus revealed, and are tentatively attributed to specific interfaces formed between a polymer-enriched skin layer and recessed acceptor aggregates. Decay time constant images of the negative SPV component confirm that the acceptor clusters act as electron-trapping centres. Whatever the photovoltaic technology, our results exemplify how some of the SPV components may remain completely hidden to conventional SPV imaging by KPFM, with possible consequences in terms of photo-response misinterpretation. This work furthermore highlights the need of implementing time-resolved techniques that can provide a quantitative measurement of the time-resolved potential.

Morphological Characterization of PE Blown Films by Atomic Force Microscopy

AbstractThe properties of blown polyethylene (PE) films depend on various factors, including crystallinity, morphology, and orientation, in addition to chemical composition. It has been shown that the optical properties are strongly influenced by surface morphology. In this work, non-contact atomic force microscopy (AFM) and polarized light microscopy (PLM) were used to visualize surface and bulk morphology. Various techniques, such as surface and line roughness, surface and line fractal dimension, pair-correlation function and nearest neighbor distance distribution function, are employed to quantify the description of morphology and to compare the morphological characteristics of a number of polyethylene (PE) films of commercial interest. A comprehensive quantitative analysis of surface topography has been performed. The co-monomer of the PE resins was found to play a significant role in the formation and the orientation of spherulite-like domains. The film cross-section microstructure has been evaluated qualitatively by using both AFM and PLM. However, quantitative analysis of bulk morphology cannot be obtained due to knife effects.

Magnetic, Thermal, and Topographic Imaging with a Nanometer-Scale SQUID-On-Lever Scanning Probe

Scanning superconducting quantum interference device (SQUID) microscopy is a magnetic imaging technique combining high-field sensitivity with nanometer-scale spatial resolution. State-of-the-art SQUID-on-tip probes are now playing an important role in mapping correlation phenomena, such as superconductivity and magnetism, which have recently been observed in two-dimensional van der Waals materials. Here, we demonstrate a scanning probe that combines the magnetic and thermal imaging provided by an on-tip SQUID with the tip-sample distance control and topographic contrast of a non-contact atomic force microscope (AFM). We pattern the nanometer-scale SQUID, including its weak-link Josephson junctions, via focused ion beam milling at the apex of a cantilever coated with Nb, yielding a sensor with an effective diameter of 365 nm, field sensitivity of 9.5 $\text{nT}/\sqrt{\text{Hz}}$ and thermal sensitivity of 620 $\text{nK}/\sqrt{\text{Hz}}$, operating in magnetic fields up to 1.0 T. The resulting SQUID-on-lever is a robust AFM-like scanning probe that expands the reach of sensitive nanometer-scale magnetic and thermal imaging beyond what is currently possible.

Adsorption configurations of Co-phthalocyanine on In2O3(111)

Indium oxide offers optical transparency paired with electrical conductivity, a combination required in many optoelectronic applications. The most-stable In2O3(111) surface has a large unit cell (1.43 nm lattice constant). It contains a mixture of both bulk-like and undercoordinated O and In atoms and provides an ideal playground to explore the interaction of surfaces with organic molecules of similar size as the unit cell. Non-contact atomic force microscopy (nc-AFM), scanning tunneling microscopy (STM), and density functional theory (DFT) were used to study the adsorption of Co-phthalocyanine (CoPc) on In2O3(111). Isolated CoPc molecules adsorb at two adsorption sites in a ∼7:3 ratio. The Co atom sits either on top of a surface oxygen (‘F configuration’) or indium atom (‘S configuration’). This subtle change in adsorption site induces different bending of the molecules and differences in their electronic structure. According to DFT, the lowest unoccupied molecular orbital of the undistorted gas-phase CoPc remains mostly unaffected in the F configuration but is filled by one electron in S configuration. At coverages up to one CoPc molecule per substrate unit cell, a mixture of domains with molecules in F and S configuration are found. Molecules in F configuration first condense into a poorly ordered F-(2 × 2) structure and finally rearrange into an F-(1 × 1) structure in which the molecules partially overlap. The only ordered structure of molecules adsorbed in S configuration is an S-(1 × 1) superstructure at full monolayer coverage.

On‐surface synthesis of porous graphene nanoribbons containing nonplanar [14]annulene pores

AbstractThe precise introduction of nonplanar pores in the backbone of graphene nanoribbon represents a great challenge. Here, we explore a synthetic strategy toward the preparation of nonplanar porous graphene nanoribbon from a predesigned dibromohexabenzotetracene monomer bearing four cove‐edges. Successive thermal annealing steps of the monomers indicate that the dehalogenative aryl‐aryl homocoupling yields a twisted polymer precursor on a gold surface and the subsequent cyclodehydrogenation leads to a defective porous graphene nanoribbon containing nonplanar [14]annulene pores and five‐membered rings as characterized by scanning tunneling microscopy and noncontact atomic force microscopy. Although the C–C bonds producing [14]annulene pores are not achieved with high yield, our results provide new synthetic perspectives for the on‐surface growth of nonplanar porous graphene nanoribbons.

Submolecular-resolution imaging of water clusters on the Cu(110) surface by atomic force microscopy

<p indent="0mm">Water/solid interfaces are important to a wide range of scientific and technological processes such as ice nucleation and growth, corrosion, lubrication, heterogeneous catalysis and electrochemistry. It is a key issue to understand the structure and dynamics of water on solid surfaces. However, the atomic-scale characterization of the weakly bonded water clusters on metal surfaces remains challenging. Here, we report submolecular-resolution imaging of the water clusters on the Cu(110) surface using a qPlus-based noncontact atomic force microscope with a CO tip. The configurations of water clusters, including the detailed OH directionalities, could be identified unambiguously by probing the weak high-order electrostatic force between the CO-tip and polar water molecules, which is further confirmed by theoretical simulations. Water clusters composed of 4, 5, and 6 water molecules are observed on the Cu(110) surface, which are mainly built from cyclic tetramers and pentamers. We notice that water hexamers contain three different types of H-bonding configurations, one cyclic tetramer with two additional water molecules adsorbed at <italic>ortho</italic>- or <italic>para</italic>-sites of the tetramer and one pentagonal ring with one additional water molecule. More interestingly, these structures could be interconverted by the tip manipulation, because of the similar adsorption energies. The isolated pentamer would act as a nucleation core for the growth of 1D pentagonal chains on the Cu(110) surface.

Experimental analysis of tip vibrations at higher eigenmodes of QPlus sensors for atomic force microscopy

QPlus sensors are non-contact atomic force microscope probes constructed from a quartz tuning fork and a tungsten wire with an electrochemically etched tip. These probes are self-sensing and offer an atomic-scale spatial resolution. Therefore, qPlus sensors are routinely used to visualize the chemical structure of adsorbed organic molecules via the so-called bond imaging technique. This is achieved by functionalizing the AFM tip with a single CO molecule and exciting the sensor at the first vertical cantilever resonance mode. Recent work using higher-order resonance modes has also resolved the chemical structure of single organic molecules. However, in these experiments, the image contrast can differ significantly from the conventional bond imaging contrast, which was suspected to be caused by unknown vibrations of the tip. This work investigates the source of these artefacts by using a combination of mechanical simulation and laser vibrometry to characterize a range of sensors with different tip wire geometries. The results show that increased tip mass and length cause increased torsional rotation of the tuning fork beam due to the off-center mounting of the tip wire, and increased flexural vibration of the tip. These undesirable motions cause lateral deflection of the probe tip as it approaches the sample, which is rationalized to be the cause of the different image contrast. The results also provide a guide for future probe development to reduce these issues.

Formation of Unconventional Stoichiometric Na-Cl Magic-Number Nanoclusters and 2D Assembly on Ir(111).

Sodium chlorides in non-1:1 stoichiometry are counterintuitive but recently their existence has been found under the high pressure condition or in the confined space between graphene sheets. Here the direct observation of the formation of Na3 Cl nanoclusters, a stable magic-number structure, is reported on an Ir(111) surface using scanning tunneling microscopy and noncontact atomic force microscopy. The stability of Na3 Cl nanoclusters in the free and adsorbed state is corroborated by density functional theory calculations. It is also found that a density of nanoclusters together with Cl adatoms may further aggregate and self-assemble into a Na3 Cl4 monolayer, forming a novel metastable phase of NaCl(111) with a honeycomb lattice. Further calculations suggest that charge transfer between the polar nanoclusters and the metal substrate stabilizes NaCl of non-1:1 stoichiometry. The work exhibits the possibility of exploring unconventional ionic crystals on the surface with atomically precise control of structure and composition.

Defect-Induced π-Magnetism into Non-Benzenoid Nanographenes.

The synthesis of nanographenes (NGs) with open-shell ground states have recently attained increasing attention in view of their interesting physicochemical properties and great prospects in manifold applications as suitable materials within the rising field of carbon-based magnetism. A potential route to induce magnetism in NGs is the introduction of structural defects, for instance non-benzenoid rings, in their honeycomb lattice. Here, we report the on-surface synthesis of three open-shell non-benzenoid NGs (A1, A2 and A3) on the Au(111) surface. A1 and A2 contain two five- and one seven-membered rings within their benzenoid backbone, while A3 incorporates one five-membered ring. Their structures and electronic properties have been investigated by means of scanning tunneling microscopy, noncontact atomic force microscopy and scanning tunneling spectroscopy complemented with theoretical calculations. Our results provide access to open-shell NGs with a combination of non-benzenoid topologies previously precluded by conventional synthetic procedures.

Chiral structures of 6,12-dibromochrysene on Au(111) and Cu(111) surfaces

Nanoscale low-dimensional chiral architectures are increasingly receiving scientific interest, because of their potential applications in many fields such as chiral recognition, separation and transformation. Using 6,12-dibromochrysene (DBCh), we successfully constructed and characterized the large-area two-dimensional chiral networks on Au(111) and one-dimensional metal-liganded chiral chains on Cu(111) respectively. The reasons and processes of chiral transformation of chiral networks on Au(111) were analyzed. We used scanning tunneling spectroscopy (STS) to analyze the electronic state information of this chiral structure. This work combines scanning tunneling microscopy (STM) with non-contact atomic force microscopy (nc-AFM) techniques to achieve ultra-high-resolution characterization of chiral structures on low-dimensional surfaces, which may be applied to the bond analysis of functional nanofilms. Density functional theory (DFT) was used to simulate the adsorption behavior of the molecular and energy analysis in order to verify the experimental results.

Chemical bond imaging using torsional and flexural higher eigenmodes of qPlus sensors.

Non-contact atomic force microscopy (AFM) with CO-functionalized tips allows visualization of the chemical structure of adsorbed molecules and identify individual inter- and intramolecular bonds. This technique enables in-depth studies of on-surface reactions and self-assembly processes. Herein, we analyze the suitability of qPlus sensors, which are commonly used for such studies, for the application of modern multifrequency AFM techniques. Two different qPlus sensors were tested for submolecular resolution imaging via actuating torsional and flexural higher eigenmodes and via bimodal AFM. The torsional eigenmode of one of our sensors is perfectly suited for performing lateral force microscopy (LFM) with single bond resolution. The obtained LFM images agree well with images from the literature, which were scanned with customized qPlus sensors that were specifically designed for LFM. The advantage of using a torsional eigenmode is that the same molecule can be imaged either with a vertically or laterally oscillating tip without replacing the sensor simply by actuating a different eigenmode. Submolecular resolution is also achieved by actuating the 2nd flexural eigenmode of our second sensor. In this case, we observe particular contrast features that only appear in the AFM images of the 2nd flexural eigenmode but not for the fundamental eigenmode. With complementary laser Doppler vibrometry measurements and AFM simulations we can rationalize that these contrast features are caused by a diagonal (i.e. in-phase vertical and lateral) oscillation of the AFM tip.

MoS2 oxidative etching caught in the act: formation of single (MoO3) n molecules.

We report the presence of sub-nm MoO x clusters formed on basal planes of the 2H MoS2 crystals during thermal oxidative etching in air at a temperature of 370 °C. Using high resolution non-contact atomic force microscopy (AFM) we provide a histogram of their preferred heights. The AFM results combined with density functional theory (DFT) simulations show remarkably well that the MoO x clusters are predominantly single MoO3 molecules and their dimers at the sulfur vacancies. Additional Raman spectroscopy, and energy and wavelength dispersive X-ray spectroscopies as well as Kelvin probe AFM investigations confirmed the presence of the MoO3/MoO x species covering the MoS2 surface only sparsely. The X-ray absorption near edge spectroscopy data confirm the MoO3 stoichiometry. Taken together, our results show that oxidative etching and removal of Mo atoms at the atomic level follow predominantly via formation of single MoO3 molecules. Such findings confirm the previously only proposed oxidative etching stoichiometry.

On-Surface Synthesis of sp-Carbon Nanostructures.

The on-surface synthesis of carbon nanostructures has attracted tremendous attention owing to their unique properties and numerous applications in various fields. With the extensive development of scanning tunneling microscope (STM) and noncontact atomic force microscope (nc-AFM), the on-surface fabricated nanostructures so far can be characterized on atomic and even single-bond level. Therefore, various novel low-dimensional carbon nanostructures, challenging to traditional solution chemistry, have been widely studied on surfaces, such as polycyclic aromatic hydrocarbons, graphene nanoribbons, nanoporous graphene, and graphyne/graphdiyne-like nanostructures. In particular, nanostructures containing sp-hybridized carbons are of great advantage for their structural linearity and small steric demands as well as intriguing electronic and mechanical properties. Herein, the recent developments of low-dimensional sp-carbon nanostructures fabricated on surfaces will be summarized and discussed.