Interplay between Energy-Level Position and Charging Effect of Manganese Phthalocyanines on an Atomically Thin Insulator

Two-Level Electronic Switching in Individual Manganese-Phthalocyanine Molecules with Jahn-Teller Distortion

Understanding single molecular switches is a crucial step in designing and optimizing molecular electronic devices with highly nonlinear functionalities, e.g., gate voltage-dependent current switching. An atomically thin insulating template, in combination with scanning probe techniques, is an ideal platform to study such switches on the single-molecule level. In this study, we investigate manganese-phthalocyanine (MnPc) molecules on monolayer-thin epitaxial hexagonal boron nitride (h-BN) on Rh(111) by scanning tunneling microscopy (STM), spectroscopy (STS), and theoretical calculations. Several interesting phenomena are found: (1) high-resolution STM imaging of the molecular orbitals reveals symmetry breaking from D4h to D2h, observed in one type of MnPc. By comparison with simulations, this phenomenon can be attributed to the Jahn-Teller effect due to the negative charging of the molecule. (2) Ambipolar transitions at the molecule occur at fixed sample biases of about ±0.4 V, which manifest as negative differential conductance signatures in dI/dV spectroscopy. (3) The stochastic two-level switching, resulting in telegraphic noise in the tunneling current, manifests as a one-electron activated process. We present a two-level switching model to accurately describe a bias-dependent current-driven transition between the levels and reveal a first-order transition. The understanding and tailoring of molecular switches on the ultrathin insulating layer will be very helpful for future organic electronics design and application.

Conformational Tuning of Magnetic Interactions in Coupled Nanographenes

Phenalenyl (C13H9) is an open-shell spin-1/2 nanographene. Using scanning tunneling microscopy (STM) inelastic electron tunneling spectroscopy (IETS), covalently bonded phenalenyl dimers have been shown to feature conductance steps associated with singlet-triplet excitations of a spin-1/2 dimer with antiferromagnetic exchange. Here, we address the possibility of tuning the magnitude of the exchange interactions by varying the dihedral angle between the two molecules within a dimer. Theoretical methods ranging from density functional theory calculations to many-body model Hamiltonians solved within different levels of approximation are used to explain STM-IETS measurements of phenalenyl dimers on a hexagonal boron nitride (h-BN)/Rh(111) surface, which exhibit signatures of twisting. By means of first-principles calculations, we also propose strategies to induce sizable twist angles in surface-adsorbed phenalenyl dimers via functional groups, including a photoswitchable scheme. This work paves the way toward tuning magnetic couplings in carbon-based spin chains and two-dimensional lattices.

Selective Formation of Homochiral Dimers by Intermolecular Charge Transfer on a hBN Nanomesh

In this study, a comprehensive characterization was conducted on a chiral starburst molecule (C57H48N4, SBM) using scanning tunneling microscopy. When adsorbed onto the hBN/Rh(111) nanomesh, these molecules demonstrate homochiral recognition, leading to a selective formation of homochiral dimers. Further tip manipulation experiments reveal that the chiral dimers are stable and primarily controlled by strong intermolecular interactions. Density functional theory (DFT) calculations supported that the chiral recognition of SBM molecules is governed by the intermolecular charge transfer mechanism, different from the common steric hindrance effect. This study emphasizes the importance of intermolecular charge transfer interactions, offering valuable insights into the chiral recognition of a simple bimolecular system. These findings hold significance for the future advancement in chirality-based electronic sensors and pharmaceuticals, where the chirality of molecules can impact their properties.

A van der Waals Heterostructure with an Electronically Textured Moiré Pattern: PtSe2/PtTe2

The interlayer interaction in Pt-dichalcogenides strongly affects their electronic structures. The modulations of the interlayer atom-coordination in vertical heterostructures based on these materials are expected to laterally modify these interlayer interactions and thus provide an opportunity to texture the electronic structure. To determine the effects of local variation of the interlayer atom coordination on the electronic structure of PtSe₂, van der Waals heterostructures of PtSe₂ and PtTe₂ have been synthesized by molecular beam epitaxy. The heterostructure forms a coincidence lattice with 13 unit cells of PtSe₂ matching 12 unit cells of PtTe₂, forming a moiré superstructure. The interaction with PtTe₂ reduces the band gap of PtSe₂ monolayers from 1.8 eV to 0.5 eV. While the band gap is uniform across the moiré unit cell, scanning tunneling spectroscopy and dI/dV mapping identify gap states that are localized within certain regions of the moiré unit cell. Deep states associated with chalcogen p_z-orbitals at binding energies of ∼ -2 eV also exhibit lateral variation within the moiré unit cell, indicative of varying interlayer chalcogen interactions. Density functional theory calculations indicate that local variations in atom coordination in the moiré unit cell cause variations in the charge transfer from PtTe₂ to PtSe₂, thus affecting the value of the interface dipole. Experimentally this is confirmed by measuring the local work function by field emission resonance spectroscopy, which reveals a large work function modulation of ∼0.5 eV within the moiré structure. These results show that the local coordination variation of the chalcogen atoms in the PtSe₂/PtTe₂ van der Waals heterostructure induces a nanoscale electronic structure texture in PtSe₂.

Ullmann-Like Covalent Bond Coupling without Participation of Metal Atoms

Ullmann-like on-surface synthesis is one of the most appropriate approaches for the bottom-up fabrication of covalent organic nanostructures and many successes have been achieved. The Ullmann reaction requires the oxidative addition of a catalyst (a metal atom in most cases): the metal atom will insert into a carbon-halogen bond, forming organometallic intermediates, which are then reductively eliminated and form C-C covalent bonds. As a result, traditional Ullmann coupling involves reactions of multiple steps, making it difficult to control the final product. Moreover, forming the organometallic intermediates will potentially poison the metal surface catalytic reactivity. In the study, we used the 2D hBN, an atomically thin sp²-hybridized sheet with a large band gap, to protect the Rh(111) metal surface. It is an ideal 2D platform to decouple the molecular precursor from the Rh(111) surface while maintaining the reactivity of Rh(111). We realize an Ullmann-like coupling of a planar biphenylene-based molecule, i.e., 1,8-dibromobiphenylene (BPBr₂), on an hBN/Rh(111) surface with an ultrahigh selectivity of the biphenylene dimer product, containing 4-, 6-, and 8-membered rings. The reaction mechanism, including electron wave penetration and the template effect of the hBN, is elucidated by combining low-temperature scanning tunneling microscopy and density functional theory calculations. Our findings are expected to play an essential role regarding the high-yield fabrication of functional nanostructures for future information devices.

Actinide Coordination Chemistry on Surfaces: Synthesis, Manipulation, and Properties of Thorium Bis(porphyrinato) Complexes

Actinide-based metal-organic complexes and coordination architectures encompass intriguing properties and functionalities but are still largely unexplored on surfaces. We introduce the in situ synthesis of actinide tetrapyrrole complexes under ultrahigh-vacuum conditions, on both a metallic support and a 2D material. Specifically, exposure of a tetraphenylporphyrin (TPP) multilayer to an elemental beam of thorium followed by a temperature-programmed reaction and desorption of surplus molecules yields bis(porphyrinato)thorium (Th(TPP)₂) assemblies on Ag(111) and hexagonal boron nitride/Cu(111). A multimethod characterization including X-ray photoelectron spectroscopy, scanning tunneling microscopy, temperature-programmed desorption, and complementary density functional theory modeling provides insights into conformational and electronic properties. Supramolecular assemblies of Th(TPP)₂ as well as individual double-deckers are addressed with submolecular precision, e.g., demonstrating the reversible rotation of the top porphyrin in Th(TPP)₂ by molecular manipulation. Our findings thus demonstrate prospects for actinide-based functional nanoarchitectures.

Precise Spin Manipulation of Single Molecule Positioning on Graphene by Coordination Chemistry

Precise spin manipulation of single molecules is crucial for future molecular spintronics. However, it has been a formidable challenge due to the complexities of the strong molecule-substrate coupling as well as the response of the molecule to external stimulus. Here we demonstrate by density functional theory calculations that precise spin manipulation can be achieved by extra CO and NO molecules coordination to transition metal phthalocyanine (TMPc) (TM = Co, Fe, Mn) molecules deposited on metal-supported graphene; the spins of TMPc molecules are switched from S to S - ¹/₂ (|S - 1|) after NO (CO) coordination. With the aid of a combination of molecular orbitals (MO) theory and recently developed principal interacting spin-orbital (PISO) analysis, the impacts of NO and CO coordinations on both adsorption configuration and spin polarization of TMPc are well elucidated. We reveal the different coordination geometries that CO always coordinates axially to the TM center with a linear geometry, while NO prefers a bent geometry, which can be attributed to the competition between the σ- and π-type interactions according to the PISO analysis. Particularly, the NO-MnPc complex adopts a bent geometry deviating from the prediction by the existing Enemark-Feltham formalism. In addition, MO analysis suggests that during the CO coordination, the simultaneous existence of σ-donation and π-back-donation promotes electrons flowing from the d_z² to partially occupied d_π (d_xz and d_xz) orbitals with subsequent reordering of the TM d-orbitals, resulting in the spin transition of S → |S - 1|. In comparison, given that NO is regarded as NO^- when it adopts a bent geometry coordinating to the TM center, the complete (CoPc) or partial (FePc and MnPc) quenching of the molecular spins caused by NO coordination is attributed to the electron transfer from TM to NO. These theoretical findings provide important insights into relevant experiments and offer an effective design strategy to realize underlying single-molecular spintronics devices integrated with two-dimensional materials.

Scanning tunneling microscopy and spectroscopy of rubrene on clean and graphene-covered metal surfaces

Rubrene (C₄₂H₂₈) was adsorbed with submonolayer coverage on Pt(111), Au(111), and graphene-covered Pt(111). Adsorption phases and vibronic properties of C₄₂H₂₈ consistently reflect the progressive reduction of the molecule-substrate hybridization. Separate C₄₂H₂₈ clusters are observed on Pt(111) as well as broad molecular resonances. On Au(111) and graphene-covered Pt(111) compact molecular islands with similar unit cells of the superstructure characterize the adsorption phase. The highest occupied molecular orbital of C₄₂H₂₈ on Au(111) exhibits weak vibronic progression while unoccupied molecular resonances appear with a broad line shape. In contrast, vibronic subbands are present for both frontier orbitals of C₄₂H₂₈ on graphene. They are due to different molecular vibrational quanta with distinct Huang-Rhys factors.

Dissimilar Decoupling Behavior of Two-Dimensional Materials on Metal Surfaces

The efficiency of hexagonal boron nitride and graphene to separate the hydrocarbon molecule C₆₄H₃₆ from Ru(0001) and Pt(111) surfaces is explored in low-temperature scanning tunneling microscopy and spectroscopy experiments. Both 2D materials enable the observation of the Franck-Condon effect in both frontier orbitals. On hexagonal boron nitride, vibronic progression with two vibrational energies gives rise to sharp orbital sidebands that are clearly visible up to the second order of the vibrational quantum number with different Huang-Rhys factors. In contrast, on graphene, orbital and vibronic spectroscopic signatures exhibit broad line shapes, with the second-order progression being hardly discriminable. Only a single vibrational quantum energy leaves its fingerprint in the Franck-Condon spectrum.

Adsorption of Azobenzene on Hexagonal Boron Nitride Nanomesh Supported by Rh(111)

Adsorption properties of azobenzene, the prototypical molecular switch, were investigated on a hexagonal boron nitride (h-BN) monolayer (“nanomesh”) prepared on Rh(111). The h-BN layer was produced by decomposing borazine (B₃N₃H₆) at 1000–1050 K. Temperature-programmed desorption (TPD) studies revealed that azobenzene molecules adsorbed on the “wire” and “pore” regions desorb at slightly different temperatures. Angle-resolved high-resolution electron energy loss spectroscopy (HREELS) measurements demonstrated that the first molecular layer is characterized predominantly by an adsorption geometry with the molecular plane parallel to the surface. Scanning tunneling microscopy (STM) indicated a clear preference for adsorption in the pores, manifesting a templating effect, but in some cases one-dimensional molecular stripes also form, implying attractive molecule–molecule interaction. Density functional theory (DFT) calculations provided further details regarding the adsorption energetics and bonding and confirmed the experimental findings that the molecules adsorb with the phenyl rings parallel to the surface, preferentially in the pores, and indicated also the presence of an attractive molecule–molecule interaction.

Self-assembly and spectroscopic fingerprints of photoactive pyrenyl tectons on hBN/Cu(111)

The controlled modification of electronic and photophysical properties of polycyclic aromatic hydrocarbons by chemical functionalization, adsorption on solid supports, and supramolecular organization is the key to optimize the application of these compounds in (opto)electronic devices. Here, we present a multimethod study comprehensively characterizing a family of pyridin-4-ylethynyl-functionalized pyrene derivatives in different environments. UV-vis measurements in toluene solutions revealed absorption at wavelengths consistent with density functional theory (DFT) calculations, while emission experiments showed a high fluorescence quantum yield. Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements of the pyrene derivatives adsorbed on a Cu(111)-supported hexagonal boron nitride (hBN) decoupling layer provided access to spatially and energetically resolved molecular electronic states. We demonstrate that the pyrene electronic gap is reduced with an increasing number of substituents. Furthermore, we discuss the influence of template-induced gating and supramolecular organization on the energies of distinct molecular orbitals. The selection of the number and positioning of the pyridyl termini in tetrasubstituted, trans- and cis-like-disubstituted derivatives governed the self-assembly of the pyrenyl core on the nanostructured hBN support, affording dense-packed arrays and intricate porous networks featuring a kagome lattice.

Interfacial charge transfer enhancement via formation of binary molecular assemblies on electronically corrugated boron nitride

Using scanning tunneling microscopy/spectroscopy (STM/STS) in conjunction with finite element simulation, we investigate the interfacial behaviors in single-component zinc phthalocyanine (ZnPc) and hexadecafluorinated zinc phthalocyanine (F16ZnPc) molecular overlayers as well as their 1 : 1 mixed-phase superstructures on h-BN/Cu(111). We show that the formation of the binary molecular superstructure drastically increases the charge transfer between F16ZnPc molecules and the substrate, which is attributed to the greater electrostatic stability of the binary assembly compared to that of the pure phase. This study highlights the significant complication in the design of donor-acceptor molecular thin films as the presence of the substrate, even a weakly interacting one, such as h-BN/metal, can still perturb the intermolecular charge transfer and thereby the physical behaviors of the hybrid system via interfacial processes.

Electric Field Control of Molecular Charge State in a Single-Component 2D Organic Nanoarray

Quantum dots (QD) with electric-field-controlled charge state are promising for electronics applications, e.g., digital information storage, single-electron transistors, and quantum computing. Inorganic QDs consisting of semiconductor nanostructures or heterostructures often offer limited control on size and composition distribution as well as low potential for scalability and/or nanoscale miniaturization. Owing to their tunability and self-assembly capability, using organic molecules as building nanounits can allow for bottom-up synthesis of two-dimensional (2D) nanoarrays of QDs. However, 2D molecular self-assembly protocols are often applicable on metals surfaces, where electronic hybridization and Fermi level pinning can hinder electric-field control of the QD charge state. Here, we demonstrate the synthesis of a single-component self-assembled 2D array of molecules [9,10-dicyanoanthracene (DCA)] that exhibit electric-field-controlled spatially periodic charging on a noble metal surface, Ag(111). The charge state of DCA can be altered (between neutral and negative), depending on its adsorption site, by the local electric field induced by a scanning tunneling microscope tip. Limited metal-molecule interactions result in an effective tunneling barrier between DCA and Ag(111) that enables electric-field-induced electron population of the lowest unoccupied molecular orbital (LUMO) and, hence, charging of the molecule. Subtle site-dependent variation of the molecular adsorption height translates into a significant spatial modulation of the molecular polarizability, dielectric constant, and LUMO energy level alignment, giving rise to a spatially dependent effective molecule-surface tunneling barrier and likelihood of charging. This work offers potential for high-density 2D self-assembled nanoarrays of identical QDs whose charge states can be addressed individually with an electric field.

Electronic Decoupling of Organic Layers by a Self-Assembled Supramolecular Network on Au(111)

A cyanuric acid and melamine (CA·M) supramolecular network, prepared via the drop-casting method under ambient conditions, can be utilized as a spacer layer to decouple electronic interactions between upper organics and the metal substrate. Typical semiconducting organics 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and C₆₀ are deposited on the CA·M network under ultrahigh vacuum conditions, forming an organics/CA·M/metal heterosystem. Both geometric and electronic structures of the upper organics are characterized by using scanning tunneling microscopy/spectroscopy (STM/STS). On the CA·M network, PTCDA molecules form a well-ordered herringbone structure in submonolayer patterns, whereas C₆₀ molecules aggregate into multilayered islands. STS spectra reveal that the energy gap between the highest occupied and the lowest unoccupied molecular orbitals (HOMO - LUMO) is 3.6 eV for PTCDA and 3.8 eV for the first layer of C₆₀ on CA·M. The remarkable bandgap broadening compared with the metal-organic contact indicates successful electronic decoupling of the upper molecules from the metal surface due to the CA·M network.

Implementing Functionality in Molecular Self-Assembled Monolayers

The planar heterocyclic molecules 1,6,7,12-tetraazaperylene on a Ag(111) metal substrate show different charging characteristics depending on their local environment: next to vacancies in self-assembled islands, molecules can be charged by local electric fields, whereas their charge state is fixed otherwise. This enables the activation of selected molecules inside islands by vacancy creation from scanning-probe-based manipulation. This concept allows for combining the precise mutual atomic-scale alignment of molecules by self-assembly, on one hand, and the implementation of specific functionality into otherwise homogeneous monolayers, on the other. Activated molecules in the direct neighborhood influence each other in their charging characteristics, suggesting their use as molecular quantum cellular automata. Surprisingly, only very few interacting molecules exhibit a rich spectroscopic signature, which offers the prospect of implementing complex functionality in such structures in the future.

Interface properties of CoPc and CoPcF16 on graphene/nickel: influence of germanium intercalation

Photoelectron spectroscopy was used to investigate electronic interface properties and interactions of the organic semiconductors CoPc and CoPcF₁₆ on graphene/nickel based substrates. Additional focus was put on the influence of germanium intercalation of graphene/nickel. The presented results demonstrate that germanium can decouple graphene from nickel and in this manner restore its buffer layer properties. No charge transfer from the substrate to the organic layer is observed in the germanium intercalated case, while interface related peaks in the Co 2p core level spectra indicate such charge transfer on graphene/nickel. Strong interface dipoles are found for CoPcF₁₆ on graphene/nickel and on germanium intercalated graphene/nickel. Fluorine Auger parameters have been measured, and the results provide evidence for polarization and charge transfer screening effects of different amounts at the unlike film-substrate interfaces. The various contributions to the observed shifts are discussed.

Electronic and magnetic properties of CoPc and FePc molecules on graphene: the substrate, defect, and hydrogen adsorption effects

Transition metal phthalocyanines (TMPcs) are particularly appealing for spintronic processing and data storage devices due to their structural simplicity and functional flexibility. To realize effective control of the spins in TMPc-based systems, it is necessary to quantify how the structural and chemical environment of the molecule affects its spin center. Herein we perform a detailed investigation of the electronic and spintronic properties of vertically stacked heterostructures formed by CoPc or FePc adsorbed on a monolayer of graphene under the influences of the gold substrate, vacancies in graphene, and extra atomic hydrogen coordination on the TMPc. By using density functional theory (DFT), we reveal that both the TMPc molecules prefer the carbon-top position on graphene, and the existence of the Au substrate enhances the stability of the adsorption, while this enhanced adsorption will not modify the molecular magnetism, keeping it the same value as in the free standing case. Moreover, with the aid of a combination of DFT and ab initio wavefunction-based calculations, our results indicate that the magnetic anisotropy of the FePc-graphene complex can be actively tuned by the Au substrate. Our calculations also show that defects in graphene including single and double vacancies can modify the magnetism of these heterostructures. In particular, the spin state of FePc can be tuned from S = 1 to S = 2 with such defect engineering. Further spin state tunability can be achieved from a hydrogenation process, with the coordination of one extra hydrogen on the Co-top site for CoPc and the pyridinic N site for FePc, respectively, tuning their spin states from S = 1/2 to S = 0 and from S = 1 to S = 2. These findings may prove to be instrumental for rational design of future molecular spintronics devices integrated with two-dimensional materials.

Exciting vibrons in both frontier orbitals of a single hydrocarbon molecule on graphene

Vibronic excitations in molecules are key to the fundamental understanding of the interaction between vibrational and electronic degrees of freedom. In order to probe the genuine vibronic properties of a molecule even after its adsorption on a surface appropriate buffer layers are of paramount importance. Here, vibrational progression in both molecular frontier orbitals is observed with submolecular resolution on a graphene-covered metal surface using scanning tunnelling spectroscopy. Accompanying calculations demonstrate that the vibrational modes that cause the orbital replica in the progression share the same symmetry as the electronic states they couple to. In addition, the vibrational progression is more pronounced for separated molecules than for molecules embedded in molecular assemblies. The entire vibronic spectra of these molecular species are moreover rigidly shifted with respect to each other. This work unravels intramolecular changes in the vibronic and electronic structure owing to the efficient reduction of the molecule-metal hybridization by graphene.

Quantitative determination of a model organic/insulator/metal interface structure

By combining X-ray photoelectron spectroscopy, X-ray standing waves and scanning tunneling microscopy, we investigate the geometric and electronic structure of a prototypical organic/insulator/metal interface, namely cobalt porphine on monolayer hexagonal boron nitride (h-BN) on Cu(111). Specifically, we determine the adsorption height of the organic molecule and show that the original planar molecular conformation is preserved in contrast to the adsorption on Cu(111). In addition, we highlight the electronic decoupling provided by the h-BN spacer layer and find that the h-BN-metal separation is not significantly modified by the molecular adsorption. Finally, we find indication of a temperature dependence of the adsorption height, which might be a signature of strongly-anisotropic thermal vibrations of the weakly bonded molecules.

Initiating and Monitoring the Evolution of Single Electrons Within Atom-Defined Structures

Using a noncontact atomic force microscope, we track and manipulate the position of single electrons confined to atomic structures engineered from silicon dangling bonds on the hydrogen terminated silicon surface. An attractive tip surface interaction mechanically manipulates the equilibrium position of a surface silicon atom, causing rehybridization that stabilizes a negative charge at the dangling bond. This is applied to controllably switch the charge state of individual dangling bonds. Because this mechanism is based on short range interactions and can be performed without applied bias voltage, we maintain both site-specific selectivity and single-electron control. We extract the short range forces involved with this mechanism by subtracting the long range forces acquired on a dimer vacancy site. As a result of relaxation of the silicon lattice to accommodate negatively charged dangling bonds, we observe charge configurations of dangling bond structures that remain stable for many seconds at 4.5 K. Subsequently, we use charge manipulation to directly prepare the ground state and metastable charge configurations of dangling bond structures composed of up to six atoms.

Surface Nanostructure Formation and Atomic-Scale Templates for Nanodevices

The holy grail in nanoelectronics is the construction of nanodevices with high density, low cost, and high performance per device and per integrated circuit. One approach is the fabrication of surface nanostructures and atomic-scale templates via the autonomous assembly of atoms and/or molecules on well-defined surfaces. To steer the atomic or molecular growth processes and create a wide range of surface nanostructures with desired properties, a comprehensive understanding of the mechanisms that control the surface self-assembly processes is required. The capability to manipulate the nanodevices at the submolecular level with good controllability is also of paramount importance. This review highlights some key recent developments in the fabrication of low-dimensional nanostructures based on supramolecular self-assembly on predefined surfaces, with particular emphasis on the rapidly expanding field of two-dimensional materials. Special attention is also given to the latest progress in single-molecule manipulation for future device applications.

Detection and Manipulation of Charge States for Double-Decker DyPc2 Molecules on Ultrathin CuO Films

Charge states of lanthanide double-decker phthalocyanines complexes significantly influence their geometrical structures and magnetic properties. In this study, the charge states of single DyPc₂ molecules on an ultrathin CuO film were detected by scanning tunneling microscopy and spectroscopy in magnetic fields. Four types of adsorptions of DyPc₂ molecules on CuO were experimentally observed. Without applying voltages, two of them were positively charged with the other two at the neutral state. By controlling the sample bias, two types of neutral molecules can be switched to the positively and negatively charged states, respectively. This manipulation was not realized for the DyPc₂ cations. A way to precisely detect the molecular charge states with and without current is beneficial for the development of molecular electronics.

Molecular assembly on two-dimensional materials

Molecular self-assembly is a well-known technique to create highly functional nanostructures on surfaces. Self-assembly on two-dimensional (2D) materials is a developing field driven by the interest in functionalization of 2D materials in order to tune their electronic properties. This has resulted in the discovery of several rich and interesting phenomena. Here, we review this progress with an emphasis on the electronic properties of the adsorbates and the substrate in well-defined systems, as unveiled by scanning tunneling microscopy. The review covers three aspects of the self-assembly. The first one focuses on non-covalent self-assembly dealing with site-selectivity due to inherent moiré pattern present on 2D materials grown on substrates. We also see that modification of intermolecular interactions and molecule-substrate interactions influences the assembly drastically and that 2D materials can also be used as a platform to carry out covalent and metal-coordinated assembly. The second part deals with the electronic properties of molecules adsorbed on 2D materials. By virtue of being inert and possessing low density of states near the Fermi level, 2D materials decouple molecules electronically from the underlying metal substrate and allow high-resolution spectroscopy and imaging of molecular orbitals. The moiré pattern on the 2D materials causes site-selective gating and charging of molecules in some cases. The last section covers the effects of self-assembled, acceptor and donor type, organic molecules on the electronic properties of graphene as revealed by spectroscopy and electrical transport measurements. Non-covalent functionalization of 2D materials has already been applied for their application as catalysts and sensors. With the current surge of activity on building van der Waals heterostructures from atomically thin crystals, molecular self-assembly has the potential to add an extra level of flexibility and functionality for applications ranging from flexible electronics and OLEDs to novel electronic devices and spintronics.

Cu- and Pd-catalyzed Ullmann reaction on a hexagonal boron nitride layer

This study demonstrates that Cu and Pd can efficiently activate Ullmann reactions on inert h-BN with two distinctive reaction paths.