Graphene nanoribbons (GNRs) are highly versatile materials due to their unique electronic, magnetic, and optical properties, which can be precisely tuned by controlling their width, edge structure, and topology. We report the on-surface synthesis and characterization of a straight N = 15 armchair GNR with periodic [18]annulene nanopores (15-pGNR). It serves as a structural link between two well-established GNRs: the pristine N = 15 armchair GNR without pores (15-AGNR) and the chevron GNR (cGNR). With the addition of the 15-pGNR reported in this study, these three GNRs form a rare experimentally accessible series of ribbons, in which the evolution of electronic properties can be tracked upon progressive carving of a basic 15-AGNR: first, by creating periodic nanopores to form 15-pGNR and then by extending the pore area and producing meandering cGNR. We have designed a molecular precursor for the 15-pGNR and grown the nanoribbons on single-crystal gold substrates by on-surface synthesis in ultra-high vacuum (UHV) conditions. The atomically precise structure of 15-pGNR was confirmed by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). The band gap of 15-pGNR was studied by scanning tunneling spectroscopy (STS) and dI/dV mapping, and the occupied electronic levels were investigated by angle-resolved photoemission spectroscopy (ARPES). A theoretical and experimental comparison of 15-pGNRs, 15-AGNRs, and cGNRs demonstrates that the introduction of periodic nanopores into 15-AGNR leads to a more than 2-fold increase in its band gap. In contrast, the band gaps of 15-pGNR and cGNR differ only by about 15%. Such band gap increase can be qualitatively understood to arise from two combined effects, the periodic perforation of the graphene lattice and the confinement effect induced by the GNR width.

The scanning tunneling microscope (STM) is a powerful tool for investigating and manipulating molecules on surfaces. We demonstrate with a low-temperature STM operated at 6.2 K the controlled manipulation of ternary clusters of persistent molecular π radicals as a whole. The ternary clusters - each self-assembled from three α,γ-bisdiphenylene-β-phenylallyl (BDPA) molecules on Au(111) - maintain their natural cluster structure throughout tip-induced translation and rotation relative to the surface. Sustained and repeated dragging of radical clusters is shown to facilitate the construction of artificial assemblies of several clusters. Our results provide new opportunities for the creation and investigation of radical-based spin assemblies on surfaces.

The Cover Feature shows the organic radical molecule α,γ-bisdiphenylene-β-phenylallyl (BDPA), which exhibits long-term stability due in part to its large number of resonance forms. The singly occupied molecular orbital (SOMO, top middle) of the molecule extends only partly into the lone phenyl ring. A radical frontier density (RFD, top right) map highlights the regions with greatest radical density in red. The bottom portion depicts BDPA adsorbed on Cu(100), where it can be deposited at room temperature as single molecules. More information can be found in the Research Article by J. D. Teeter and co-workers (DOI: 10.1002/cphc.202400852).

Graphene nanoribbons (GNRs) of precise size and shape, critical for controlling electronic properties and future device applications, can be realized via precision synthesis on surfaces using rationally designed molecular precursors. Fluorine-bearing precursors have the potential to form GNRs on nonmetallic substrates suitable for device fabrication. Here, we investigate the deposition temperature-mediated growth of a new fluorine-bearing precursor, 6,11-diiodo-1,4-bis(2-fluorophenyl)-2,3-diphenyltriphenylene (C42H24F2I2), into helically shaped polymer intermediates and chevron-type GNRs on Au(111) by combining scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory simulations. The fluorinated precursors do not adsorb on the Au(111) surface at lower temperatures, necessitating an optimum substrate temperature to achieve maximum polymer and GNR lengths. We compare the adsorption behavior with that of pristine chevron precursors and discuss the effects of C-H and C-F bonds. The results elucidate the growth mechanism of GNRs with fluorine-bearing precursors and establish a foundation for future synthesis of GNRs on nonmetallic substrates. Fluorine-bearing precursors offer new opportunities for the on-surface synthesis of graphene nanoribbons, but the growth conditions are critical for achieving optimal results. Here, the authors investigate the deposition temperature-mediated growth of helically shaped polymers and graphene nanoribbons on Au(111) from an internally fluorinated precursor.

The unique spin texture of quantum states in topological materials underpins many proposed spintronic applications. However, realizations of such great potential are stymied by perturbations, such as temperature and local fields imposed by impurities and defects, that can render a promising quantum state uncontrollable. Here, we report room-temperature observation of interaction between Rashba states and topological surface states, which manifests unique spin textures controllable by layer thickness of thin films. Specifically, we combine scanning tunneling microscopy/spectroscopy with the first-principles theoretical calculation to find the robust Rashba states coexisting with topological surface states along the surface steps with characteristic spin textures in momentum space. The Rashba edge states can be switched off by reducing the thickness of a topological insulator Bi2Se3 to bolster their interaction with the hybridized topological surface states. The study unveils a manipulating mechanism of the spin textures at room temperature, reinforcing the necessity of thin film technology in controlling quantum states.

The adsorption of the radical α,ɣ-bisdiphenylene-β-phenylallyl (BDPA) molecule to the Cu(100) surface was studied using scanning tunnelling microscopy (STM), scanning tunnelling spectroscopy (STS), and density functional theory (DFT) calculations accounting for dispersion forces. BDPA on Cu(100) was observed to align preferentially along ⟨ 01 1 ‾ ⟩ ${\langle 01\bar{1}\rangle }$ directions due to weak Cu-C chemisorption between fluorenyl carbons with the underlying copper atoms. The curved shape of the BDPA molecule on Cu(100) can be ascribed to the lack of molecular orbital character on the phenyl substituent. A Kondo-like feature from differential conductance (dI/dV) measurements centered close to the Fermi energy ( E F ${{E}_{{\rm F}}}$ ) suggests the retention of an electron spin-1/2 state, which is corroborated by hybrid DFT calculations that place the SOMO (singly occupied molecular orbital) below and SUMO (singly unoccupied molecular orbital) above E F ${{E}_{{\rm F}}}$ for BDPA adsorbed to Cu(100).

The interplay of non-trivial band topology and magnetism gives rise to a series of exotic quantum phenomena, such as the emergent quantum anomalous Hall (QAH) effect and topological magnetoelectric effect. Many of these quantum phenomena have local manifestations when the global symmetry is broken. Here, we report local signatures of the thickness dependent topology in intrinsic magnetic topological insulator MnBi_2Te_4(MBT), using scanning tunneling microscopy and spectroscopy on molecular beam epitaxy grown MBT thin films. A thickness-dependent band gap with an oscillatory feature is revealed, which we reproduce with theoretical calculations. Our theoretical results indicate a topological quantum phase transition beyond a film thickness of one monolayer, with alternating QAH and axion insulating states for even and odd layers, respectively. At an even-odd layer step, a localized gapped electronic state is observed, in agreement with an axion insulator edge state that results from a phase transition across the step. The demonstration of thickness-dependent topological properties highlights the role of nanoscale control over novel quantum states, reinforcing the necessity of thin film technology in quantum information science applications.

A two-dimensional (2D) hydrogen-bonded cocrystal was synthesized from croconic acid (CA) and benzimidazole (BI) on a gold surface under ultrahigh vacuum conditions. The network domains have a 1:1 CA/BI stoichiometry, can be synthesized over a range of temperatures, and contain one-dimensional chains of molecules connected by heterogeneous hydrogen bonds. Density functional theory (DFT) computations suggest that a tautomeric salt-like structure, with deprotonated CA and protonated BI, is the most stable model, which creates heterogeneous N-H center dot center dot center dot O contacts instead of N-H center dot center dot center dot O ones. The homogeneity of the network's appearance in scanning tunneling microscopy (STM) and a habitual change in the STM features under certain tip conditions indicate that there is an equilibrium of tautomeric molecular states that may be influenced to some degree by STM stimuli. Overall, this study demonstrates how careful consideration of the precursor molecules can tune the architecture within a family of cocrystal networks and introduce desired bonding motifs that haven't been achieved by solution-based synthesis for these species, such as the heterogeneous hydrogen bonds herein.

AbstractThe on‐surface coupling of the prototypical precursor molecule for graphene nanoribbon synthesis, 6,11‐dibromo‐1,2,3,4‐tetraphenyltriphenylene (C42Br2H26, TPTP), and its non‐brominated analog hexaphenylbenzene (C42H30, HPB), was investigated on coinage metal substrates as a function of thermal treatment. For HPB, which forms non‐covalent 2D monolayers at room temperature, a thermally induced transition of the monolayer's structure could be achieved by moderate annealing, which is likely driven by π‐bond formation. It is found that the dibrominated carbon positions of TPTP do not guide the coupling if the growth occurs on a substrate at temperatures that are sufficient to initiate C−H bond activation. Instead, similar one‐dimensional molecular structures are obtained for both types of precursors, HPB and TPTP.

We report the on-surface synthesis and spectroscopic study of laterally extended chevron graphene nanoribbons (GNRs) and compare them with the established chevron GNRs, emphasizing the consistency of bandgap reduction of semiconducting GNRs with increased width. The extended chevron GNRs grown on Au(111) exhibit a bandgap of about 2.2 eV, which is considerably smaller than the values reported for chevron GNRs in similar studies.

A method for synthesizing photoactive inorganic perovskite quantum dot inks and an inkjet printer deposition method, using the synthesized inks, are demonstrated. The ink synthesis is based on a simple wet chemical reaction and the inkjet printing protocol is a facile step by step method. The inkjet printed thin films have been characterized by X-ray diffraction, optical absorption spectroscopy, photoluminescent spectroscopy, and electronic transport measurements. X-ray diffraction of the printed quantum dot films indicates a crystal structure consistent with an orthorhombic room temperature phase with (001) orientation. In conjunction with other characterization methods, the X-ray diffraction measurements show high quality films can be obtained through the inkjet printing method.

The Cover Feature illustrates the on-surface synthesis of graphene nanoribbons on Au(111). The atomically precise structure of the ribbons was confirmed by noncontact atomic force microscopy, while scanning tunneling spectroscopy revealed their bandgap of about 2.2 eV. More information can be found in the Communication by J. D. Teeter et al. on page 2281 in Issue 18, 2019 (DOI: 10.1002/cphc.201900445).

Graphene nanoribbon (GNR) heterojunctions have been synthesized by a bottom-up approach on an Au (111) substrate from a mixture of two structurally similar but visually distinct chevron-type molecular precursors. The resulting heterojunctions were composed of the units of chevron GNRs (cGNRs) and new laterally extended chevron GNRs (eGNRs), which contained an additional benzene ring. Because of their intentional visual difference, cGNR and eGNR units could be conveniently distinguished in scanning tunneling microscopy images. Differential conductance (dI/dV) mapping of GNR heterojunctions revealed differences in electronic structures of cGNRs and eGNRs. Interestingly, the characteristic conduction band states of cGNRs in the dI/dV maps were shown to be sensitive to the effects of both lateral extension of ribbons and the edge defects, emphasizing the importance of synthesizing GNRs and GNR heterojunctions with atomic precision. The dI/dV maps further showed that both effects could be localized within the corresponding GNR units even if they are bonded to structurally and electronically different neighbors, which suggests a possibility of engineering complex GNR-based electronic nanostructures with nanoscale modulation of properties. The new eGNR units could be potentially combined with other chevron-type GNRs, such as nitrogen-doped cGNRs, into a variety of new GNR heterojunctions.

Photoactive perovskite quantum dot films, deposited via an inkjet printer, have been characterized by x-ray diffraction and x-ray photoelectron spectroscopy. The crystal structure and bonding environment are consistent with CsPbBr3 perovskite quantum dots. The current-voltage (I-V) and capacitance-voltage (C-V) transport measurements indicate that the photo-carrier drift lifetime can exceed 1 ms for some printed perovskite films. This far exceeds the dark drift carrier lifetime, which is below 50 ns. The printed films show a photocarrier density 10(9) greater than the dark carrier density, making these printed films ideal candidates for application in photodetectors. The successful printing of photoactive-perovskite quantum dot films of CsPbBr3, indicates that the rapid prototyping of various perovskite inks and multilayers is realizable.

Synthesis of quantum dots is a valuable experiment for demonstration and discussion of quantum phenomena in undergraduate chemistry curricula. Recently, a new class of all inorganic perovskite quantum dots (QDs) with a formula of CsPbX3 (X = Cl, Br, I) was presented and attracted tremendous attention. Here we adapt the synthesis of CsPbX3 QDs for implementation in inorganic chemistry laboratory class. Perovskite QDs have a number of advantages: they exhibit bright photoluminescence in the visible range of spectrum with a narrow bandwidth, and their emission wavelength can be changed by tuning both size and composition of nanoparticles. The described experiment provides a discussion point on many important concepts of inorganic chemistry, materials science, and nanotechnology, such as colloidal synthesis of nanoparticles, perovskite crystal structure, quantum size effect, as well as photovoltaics and renewable energy.

Atomically precise graphene nanoribbons (GNRs) of two types, chevron GNRs and N = 7 straight armchair GNRs (7-AGNRs), have been synthesized through a direct contact transfer (DCT) of molecular precursors on Au(111) and gradual annealing. This method provides an alternative to the conventional approach for the deposition of molecules on surfaces by sublimation and simplifies preparation of dense monolayer films of GNRs. The DCT method allows deposition of molecules on a surface in their original state and then studying their gradual transformation to polymers to GNRs by scanning tunneling microscopy (STM) upon annealing. We performed STM characterization of the precursors of chevron GNRs and 7-AGNRs, and demonstrate that the assemblies of the intermediates of the GNR synthesis are stabilized by π-π interactions. This conclusion was supported by the density functional theory calculations. The resulting monolayer films of GNRs have sufficient coverage and density of nanoribbons for ex situ characterization by spectroscopic methods, such as Raman spectroscopy, and may prove useful for the future GNR device studies.

Atomically precise chevron graphene nanoribbons (GNRs) have been synthesized on Cu(111) substrates by the surface-assisted coupling of 6,11-dibromo-1,2,3,4-tetraphenyltriphenylene (C42Br2H26) and thermal cyclodehydrogenation of the resulting polymer. The GNRs form on Cu(111) epitaxially along the 〈112〉 crystallographic directions, which was found to be in agreement with the computational results, and at lower temperatures than on Au(111). This work demonstrates that the substrate plays an important role in the on-surface synthesis of GNRs and can result in new assembly modes of GNR structures.

Planar, 1D and hydrogen-bonded chains of benzimidazole molecules have been fabricated through surface-assisted self-assembly on Ag(111) and Au(111) and investigated with scanning tunneling microscopy. The hydrogen bond between the benzimidazoles and the coupling to the molecular pi-electron system, of the type -C=N center dot center dot center dot H-N-C=, which exists in bulk crystals and gives rise to ferroelectricity at room temperature, is also observed in the supported 1D chains. Inspired by this finding, the proton-transfer mechanism in 1D chains of benzimidazoles in the gas phase and on coinage metal surfaces was investigated with density functional theory (DFT) calculations. It is demonstrated that the proton transfer, which is needed to reverse the dipole moment along a model chain, is a low-energy process in the gas phase. The substrate shapes this energy barrier and lowers it as compared with free chains. A hydrogen-transfer pathway via a tautomerized state is identified, and because of the relative instability of the tautomerized state, a concerted or cascaded proton transfer along the chains seems plausible. This study predicts that 1D organic ferroelectrics based on benzimidazoles can exist if the molecule-substrate interactions are appropriately controlled.