Spatial resolution of a type II heterojunction in a single bipolar molecule

Preservation of the donor-acceptor character of a carbazole-phenalenone dyad upon adsorption on Pt(111)

Donor-acceptor molecules are a subject of great attention due to their immense potential in molecular electronics and photovoltaics. Despite numerous extensive studies demonstrating their functionality in solution, the donor-acceptor character is usually lost upon adsorption on a conducting substrate. Here the concept of breaking the conjugation between the donor and acceptor unit by insertion of a bridge is used. Furthermore, the bridge introduces a kink into the dyad and thus, reduces the possibility of hybridization with the substrate. A donor-bridge-acceptor dyad composed of carbazole and phenalenone units joined through a flexible bridge is synthesized and deposited on a Pt(111) surface. Its electronic properties are investigated with a combination of low temperature scanning tunneling microscope measurements and density functional theory simulations. Two preferential adsorption configurations are identified, in which individual molecules form strong bonds to the substrate and to a Pt adatom. Differential conductance measurements and atomistic simulations evidence the preservation of a reduced donor-acceptor character upon adsorption of the molecule, where this reduction is ascribed to the strong molecule-metal hybridization. Our results highlight the changes in donor-acceptor character of the dyad induced by the substrate and provide guidelines for the use of donor-bridge-acceptor molecules as functional units in solid-state devices.

Electrospray deposition of structurally complex molecules revealed by atomic force microscopy

Advances in organic chemistry allow the synthesis of large, complex and highly functionalized organic molecules having potential applications in optoelectronics, molecular electronics and organic solar cells. Their integration into devices as individual components or highly ordered thin-films is of paramount importance to address these future prospects. However, conventional sublimation techniques in vacuum are usually not applicable since large organic compounds are often non-volatile and decompose upon heating. Here, we prove by atomic force microscopy and scanning tunneling microscopy, the structural integrity of complex organic molecules deposited onto an Au(111) surface using electrospray ionisation deposition. High resolution AFM measurements with CO-terminated tips unambiguously reveal their successful transfer from solution to the gold surface in ultra-high vacuum without degradation of their chemical structures. Furthermore, the formation of molecular structures from small islands to large and highly-ordered self-assemblies of those fragile molecules is demonstrated, confirming the use of electrospray ionisation to promote also on-surface polymerization reactions of highly functionalized organic compounds, biological molecules or molecular magnets.

Atomically precise graphene nanoribbon heterojunctions from a single molecular precursor

The rational bottom-up synthesis of atomically defined graphene nanoribbon (GNR) heterojunctions represents an enabling technology for the design of nanoscale electronic devices. Synthetic strategies used thus far have relied on the random copolymerization of two electronically distinct molecular precursors to yield GNR heterojunctions. Here we report the fabrication and electronic characterization of atomically precise GNR heterojunctions prepared through late-stage functionalization of chevron GNRs obtained from a single precursor. Post-growth excitation of fully cyclized GNRs induces cleavage of sacrificial carbonyl groups, resulting in atomically well-defined heterojunctions within a single GNR. The GNR heterojunction structure was characterized using bond-resolved scanning tunnelling microscopy, which enables chemical bond imaging at T = 4.5 K. Scanning tunnelling spectroscopy reveals that band alignment across the heterojunction interface yields a type II heterojunction, in agreement with first-principles calculations. GNR heterojunction band realignment proceeds over a distance less than 1 nm, leading to extremely large effective fields.

Donor-Acceptor Properties of a Single-Molecule Altered by On-Surface Complex Formation

Electron donor-acceptor molecules are of outstanding interest in molecular electronics and organic solar cells for their intramolecular charge transfer controlled via electrical or optical excitation. The preservation of their electronic character in the ground state upon adsorption on a surface is cardinal for their implementation in such single-molecule devices. Here, we investigate by atomic force microscopy and scanning tunneling microscopy a prototypical system consisting of a π-conjugated tetrathiafulvalene-fused dipyridophenazine molecule adsorbed on thin NaCl films on Cu(111). Depending on the adsorption site, the molecule is found either in a nearly undisturbed free state or in a bound state. In the latter case, the molecule adopts a specific adsorption site, leading to the formation of a chelate complex with a single Na⁺ alkali cation pulled out from the insulating film. Although expected to be electronically decoupled, the charge distribution of the complex is drastically modified, leading to the loss of the intrinsic donor-acceptor character. The chelate complex formation is reversible with respect to lateral manipulations, enabling tunable donor-acceptor molecular switches activated by on-surface coordination.

Controlling Charge Carrier Overlap in Type-II ZnSe/ZnS/CdS Core-Barrier-Shell Quantum Dots

We describe the synthesis and spectroscopic characterization of colloidal ZnSe/ZnS/CdS nanocrystals, which exhibit a type-II electronic structure and wave function overlap that is strongly dependent on the thickness of the ZnS barrier. Barrier thickness is controlled by both the amount of deposited material and the reaction and annealing temperature of CdS shell growth. The results show that a single monolayer of ZnS mitigates the overlap significantly, while four and more monolayers effectively suppress band edge absorption and emission. Transient absorption spectra reveal a broad distribution of excitons with mixed S and P symmetry, which become allowed due to alloy formation and contribute to charge carrier relaxation across the barrier. We present a model of the core/shell interface based on cation diffusion, which allows one to estimate the extent of the diffusion layer from optical spectra.

Importance of excitonic effect in charge separation at quantum-dot/organic interface: first-principles many-body calculations

The staggered alignment of quasiparticle energy levels is widely regarded to be the key criterion necessary for electron-hole charge separation to occur at heterogeneous material interfaces. However, staggered energy levels at nanoscale interfaces, such as those between organic molecules and inorganic quantum dots, do not necessarily imply charge separation across the interface because the excitonic effect is often significant. Using quantum Monte Carlo calculations, we perform a detailed study of the role of the excitonic effects on charge separation across a representative set of interfaces between organic molecules and quantum dots. We find that the exciton binding energy of charge transfer excitons is significantly larger than would be estimated from a simple Coulombic analysis and, at these nanoscale interfaces, can be as significant as that of Frenkel excitons. This implies that charge transfer excitons can act as trap states and facilitate electron-hole recombination instead of charge separation. We conclude that in general, for nanoscale interfaces, high-fidelity quantum many-body calculations are essential for an accurate evaluation of the detailed energetic balance between localized and delocalized excitons and, thus, are crucial for the predictive treatment of interfacial charge separation processes.

Contrast formation in Kelvin probe force microscopy of single π-conjugated molecules

We report the contrast formation in the local contact potential difference (LCPD) measured by Kelvin probe force microscopy (KPFM) on single charge-transfer complexes (CTCs) on a NaCl bilayer on Cu(111). At different tip heights, we found quantitatively different LCPD contrasts that characterize different properties of the molecule. In the small distance regime, the tip penetrates the electron density of the molecule, and the contrast is related to the size and topography of the electron shell of the molecule. For larger distances, the LCPD contrast corresponds to the electrostatic field above the molecule. However, in the medium-distance regime, that is, for tip heights similar to the size of the molecule, the nonspherical distribution of π- and σ-electrons often conceals the effect of the partial charges within the molecule. Only for large distances does the LCPD map converge toward the simple field of a dipole for a polar molecule.

Influence of nanoparticle size to the electrical properties of naphthalenediimide on single-walled carbon nanotube wiring

Nanoparticles of N,N'-bis(n-alkyl)tetracarbonatenaphthalenediimide (NDI) were adsorbed on single-walled carbon nanotube (SWNT) wires dispersed on a SiO(2) substrate. The electrical properties were measured along the long axis of the SWNTs, and in all cases through the nanoparticles showed rectification in semiconducting I-V curve. The plateau width of the I-V curve through the NDI nanoparticles on metallic SWNTs decreased as the particle size increased, while the rectification ratio increased. The conduction mechanism was changed from tunneling conduction to Schottky-like conduction and their boundary is at about 3 nm diameter.

Imaging the charge distribution within a single molecule

Scanning tunnelling microscopy and atomic force microscopy can be used to study the electronic and structural properties of surfaces, as well as molecules and nanostructures adsorbed on surfaces, with atomic precision, but they cannot directly probe the distribution of charge in these systems. However, another form of scanning probe microscopy, Kelvin probe force microscopy, can be used to measure the local contact potential difference between the scanning probe tip and the surface, a quantity that is closely related to the charge distribution on the surface. Here, we use a combination of scanning tunnelling microscopy, atomic force microscopy and Kelvin probe force microscopy to examine naphthalocyanine molecules (which have been used as molecular switches) on a thin insulating layer of NaCl on Cu(111). We show that Kelvin probe force microscopy can map the local contact potential difference of this system with submolecular resolution, and we use density functional theory calculations to verify that these maps reflect the intramolecular distribution of charge. This approach could help to provide fundamental insights into single-molecule switching and bond formation, processes that are usually accompanied by the redistribution of charge within or between molecules.

The frontier orbitals of a push-pull azobenzene adsorbed on a metal surface in different bonding geometries investigated by scanning tunneling spectroscopy and spectroscopy mapping

The electronic structure of 4-anilino-4'-nitroazobenzene superstructures formed on Au(111) at 250 K is investigated by low temperature scanning tunneling microscopy, scanning tunneling spectroscopy, and dI/dV mapping at 5 K. Changes in the dI/dV maps of this push-pull molecule reflect the spatial distribution of the frontier orbitals on the molecular scale. Spectra of the trans- and the cis∗-isomer differ between themselves and in different parts of supramolecular assemblies. The relative importance of these differences is discussed.

An electron induced two-dimensional switch made of azobenzene derivatives anchored in supramolecular assemblies

Supramolecular assemblies of 4-anilino-4'-nitroazobenzene are investigated on the Au(111) surface by low temperature scanning tunneling microscopy and spectroscopy with submolecular resolution. Adsorption at 250 K leads to three different structures that are formed via hydrogen bonds: a star structure and two types of line structures: a meandering and a zigzag line. The formation of these supramolecular assemblies is affected by the available space on the fcc domains of the reconstructed Au(111) substrate as well as by the two-dimensional chirality of the molecules on the surface. The star structure is enantiomerically pure, while both types of lines consist of a racemic mixture. Bonding between homochiral pairs differ from the one between heterochiral pairs in the position of the hydrogen bonds. Inside these supramolecular assemblies two configurations of the molecules are identified: An almost straight trans-configuration and a slightly bent cis*-configuration. The trans-configuration largely reflects the structure of this isomer in gas phase, while the cis*-configuration is two-dimensional on the surface in contrast to the three-dimensional gas phase cis-configuration. The reversible trans-cis* isomerization is induced by electron tunneling through the LUMO+1 state of the molecule, which is located at +2.9 V.