Selective analysis of molecular states by functionalized scanning tunneling microscopy tips

Construction and physical properties of low-dimensional structures for nanoscale electronic devices

Over the past decades, construction of nanoscale electronic devices with novel functionalities based on low-dimensional structures, such as single molecules and two-dimensional (2D) materials, has been rapidly developed. To investigate their intrinsic properties for versatile functionalities of nanoscale electronic devices, it is crucial to precisely control the structures and understand the physical properties of low-dimensional structures at the single atomic level. In this review, we provide a comprehensive overview of the construction of nanoelectronic devices based on single molecules and 2D materials and the investigation of their physical properties. For single molecules, we focus on the construction of single-molecule devices, such as molecular motors and molecular switches, by precisely controlling their self-assembled structures on metal substrates and charge transport properties. For 2D materials, we emphasize their spin-related electrical transport properties for spintronic device applications and the role that interfaces among 2D semiconductors, contact electrodes, and dielectric substrates play in the electrical performance of electronic, optoelectronic, and memory devices. Finally, we discuss the future research direction in this field, where we can expect a scientific breakthrough.

Atomistic Origins of Surface Defects in CH3NH3PbBr3 Perovskite and Their Electronic Structures

The inherent instability of CH₃NH₃PbX₃ remains a major technical barrier for the industrial applications of perovskite materials. Recently, the most stable surface structures of CH₃NH₃PbX₃ have been successfully characterized by using density functional theory (DFT) calculations together with the high-resolution scanning tunneling microscopy (STM) results. The two coexisting phases of the perovskite surfaces have been ascribed to the alternate orientation of the methylammonium (MA) cations. Notably, similar surface defect images (a dark depression at the sites of X atoms) have been observed on surfaces produced with various experimental methods. As such, these defects are expected to be intrinsic to the perovskite crystals and may play an important role in the structural decomposition of perovskite materials. Understanding the nature of such defects should provide some useful information toward understanding the instability of perovskite materials. Thus, we investigate the chemical identity of the surface defects systematically with first-principles density functional theory calculations and STM simulations. The calculated STM images of the Br and Br-MA vacancies are both in good agreement with the experimental measurements. In vacuum conditions, the formation energy of Br-MA is 0.43 eV less than the Br vacancy. In the presence of solvation effects, however, the formation energy of a Br vacancy becomes 0.42 eV lower than the Br-MA vacancy. In addition, at the vacancy sites, the adsorption energies of water, oxygen, and acetonitrile molecules are significantly higher than those on the pristine surfaces. This clearly demonstrated that the structural decomposition of perovskites are much easier to start from these vacancy sites than the pristine surfaces. Combining DFT calculations and STM simulations, this work reveals the chemical identities of the intrinsic defects in the CH₃NH₃PbX₃ perovskite crystals and their effects on the stability of perovskite materials.

High-resolution scanning tunneling microscopy imaging of Si(1 1 1)-7 × 7 structure and intrinsic molecular states

We review our achievements in exploring the high resolution imaging of scanning tunneling microscopy (STM) on the surface and adsorbates in a ultra-high vacuum system, by modifying the STM tip or introducing a decoupled layer onto the substrate. With an ultra-sharp tip, the highest resolution of Si(1 1 1)-7 × 7 reconstruction can be achieved, in which all the rest atoms and adatoms are observed simultaneously with high contrast. Further functionalization of STM tips can realize selective imaging of inherent molecular states. The electronic states of perylene and metal-phthalocyanine molecules are resolved with special decorated tips on metal substrates at low temperature. Moreover, we present two kinds of buffer layer: an organic molecular layer and epitaxially grown graphene to decouple the molecular electronic structure from the influence of the underlying metallic substrate and allow the direct imaging of the intrinsic orbitals of the adsorbed molecules. Theoretical calculations and STM simulations, based on first-principle density function theory, are performed in order to understand and verify the mechanism of high-resolution images. We propose that our results provide impactful routes to pursue the goal of higher resolution, more detailed information and extensive properties for future STM applications.

Channel selective tunnelling through a nanographene assembly

We report selective tunnelling through a nanographene intermolecular tunnel junction achieved via scanning tunnelling microscope tip functionalization with hexa-peri-hexabenzocoronene (HBC) molecules. This leads to an offset in the alignment between the energy levels of the tip and the molecular assembly, resulting in the imaging of a variety of distinct charge density patterns in the HBC assembly, not attainable using a bare metallic tip. Different tunnelling channels can be selected by the application of an electric field in the tunnelling junction, which changes the condition of the HBC on the tip. Density functional theory-based calculations relate the imaged HBC patterns to the calculated molecular orbitals at certain energy levels. These patterns bear a close resemblance to the π-orbital states of the HBC molecule calculated at the relevant energy levels, mainly below the Fermi energy of HBC. This correlation demonstrates the ability of an HBC functionalized tip as regards accessing an energy range that is restricted to the usual operating bias range around the Fermi energy with a normal metallic tip at room temperature. Apart from relating to molecular orbitals, some patterns could also be described in association with the Clar aromatic sextet formula. Our observations may help pave the way towards the possibility of controlling charge transport between organic interfaces.

PTCDA on Cu(111) partially covered with NaCl

The organic molecule 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) was studied by means of scanning tunneling microscopy (STM) on thin insulating NaCl films grown on a Cu(111) single crystal. The deposition of approximately two monolayers (ML) of sodium chloride onto a Cu(111) substrate at a sample temperature of about 350 K causes a rather rough growth of (100)-oriented NaCl islands up to a local height of 4 ML. For submonolayer coverages (0.1 and 0.4 ML) of PTCDA on a Cu(111) surface partly covered with NaCl, two different rod structures of PTCDA were found on the copper surface, which are in contrast to previously published data for PTCDA on Cu(111) showing a herringbone-like arrangement. These findings can be explained by the formation of a Na(x)-PTCDA complex. On NaCl covered areas, single PTCDA molecules adsorb at vacancies of [010] and [001] oriented steps of the NaCl(100) islands. In this case, the electrostatic forces between the polar step edges and the PTCDA molecules are dominant. The terraces of the alkali halide surface are free of PTCDA molecules.

Single-molecule chemistry of metal phthalocyanine on noble metal surfaces

To develop new functional materials and nanoscale electronics, researchers would like to accurately describe and precisely control the quantum state of a single molecule on a surface. Scanning tunneling microscopy (STM), combined with first-principles simulations, provides a powerful technique for acquiring this level of understanding. Traditionally, metal phthalocyanine (MPc) molecules, composed of a metal atom surrounded by a ligand ring, have been used as dyes and pigments. Recently, MPc molecules have shown great promise as components of light-emitting diodes, field-effect transistors, photovoltaic cells, and single-molecule devices. In this Account, we describe recent research on the characterization and control of adsorption and electronic states of a single MPc molecule on noble metal surfaces. In general, the electronic and magnetic properties of a MPc molecule largely depend on the type of metal ion within the phthalocyanine ligand and the type of surface on which the molecule is adsorbed. However, with the STM technique, we can use on-site molecular "surgery" to manipulate the structure and the properties of the molecule. For example, STM can induce a dehydrogenation reaction of the MPc, which allows us to control the Kondo effect, which describes the spin polarization of the molecule and its interaction with the complex environment. A specially designed STM tip can allow researchers to detect certain molecule-surface hybrid states that are not accessible by other techniques. By matching the local orbital symmetry of the STM tip and the molecule, we can generate the negative differential resistance effect in the formed molecular junction. This orbital symmetry based mechanism is extremely robust and does not critically depend on the geometry of the STM tip. In summary, this simple model system, a MPc molecule absorbed on a noble metal surface, demonstrates the power of STM for quantum characterization and manipulation of single molecules, highlighting the potential of this technique in a variety of applications.

Homochiral recognition among organic molecules on copper(110)

The adsorption of a prochiral quinacridone derivative (QA16C) with two alkyl chains of 16 carbon atoms on a Cu(110) surface was investigated with variable-temperature scanning tunneling microscopy. QA16C molecules prefer to assemble at 150 K into short homochiral molecular lines with two enantiomorphous orientations in which the lateral alkyl chains exhibit partial disorder. With increasing sample temperatures, the QA16C lines form larger well-ordered homochiral domains. As a reason for the homochiral recognition, we identify a rigid alignment of the molecule due to the interaction with the substrate. In addition, lateral intermolecular interactions in the form of hydrogen bonding and van der Waals interactions are identified.

Anchoring of a single molecular rotor and its array on metal surfaces using molecular design and self-assembly

Functionalizing of single molecules on surfaces has manifested great potential for bottom-up construction of complex devices on a molecular scale. We discuss the growth mechanism for the initial layers of polycyclic aromatic hydrocarbons on metal surfaces and we review our recent progress on molecular machines, and present a molecular rotor with a fixed off-center axis formed by chemical bonding. These results represent important advances in molecular-based nanotechnology.

Role of lateral alkyl chains in modulation of molecular structures on metal surfaces

We use low energy electron diffraction, scanning tunneling microscopy, first-principles density-functional theory, and molecular mechanics calculations to analyze the adsorption and growth of quinacridone derivatives (QA) with alkyl chains of 4 and 16 carbon atoms on a Ag(110) substrate. Surprisingly, we find that the alkyl chains determine the orientation of the molecular overlayers. While the interaction of QA and the Ag substrate is primarily due to chemical bonding of oxygen to the silver substrate, determining the molecular orientation and preferred adsorption site, the intermolecular arrangement can be adjusted via the length of alkyl chains. We are thus able to fabricate uniform QA films with very well controlled physical properties.