@article{ma_xiao_bonnesen_liang_puretzky_huang_kolmer_sumpter_lu_hong_et al._2021, title={On-surface cyclodehydrogenation reaction pathway determined by selective molecular deuterations}, volume={11}, ISSN={["2041-6539"]}, DOI={10.1039/d1sc04908a}, abstractNote={Understanding the reaction mechanisms of dehydrogenative Caryl–Caryl coupling is the key to directed formation of π-extended polycyclic aromatic hydrocarbons. Here we utilize isotopic labeling to identify the exact pathway of cyclodehydrogenation reaction in the on-surface synthesis of model atomically precise graphene nanoribbons (GNRs). Using selectively deuterated molecular precursors, we grow seven-atom-wide armchair GNRs on a Au(111) surface that display a specific hydrogen/deuterium (H/D) pattern with characteristic Raman modes. A distinct hydrogen shift across the fjord of Caryl–Caryl coupling is revealed by monitoring the ratios of gas-phase by-products of H2, HD, and D2 with in situ mass spectrometry. The identified reaction pathway consists of a conrotatory electrocyclization and a distinct [1,9]-sigmatropic D shift followed by H/D eliminations, which is further substantiated by nudged elastic band simulations. Our results not only clarify the cyclodehydrogenation process in GNR synthesis but also present a rational strategy for designing on-surface reactions towards nanographene structures with precise hydrogen/deuterium isotope labeling patterns.}, journal={CHEMICAL SCIENCE}, author={Ma, Chuanxu and Xiao, Zhongcan and Bonnesen, Peter V and Liang, Liangbo and Puretzky, Alexander A. and Huang, Jingsong and Kolmer, Marek and Sumpter, Bobby G. and Lu, Wenchang and Hong, Kunlun and et al.}, year={2021}, month={Nov} }
 @article{ma_xiao_puretzky_wang_mohsin_huang_liang_luo_lawrie_gu_et al._2020, title={Engineering Edge States of Graphene Nanoribbons for Narrow-Band Photoluminescence}, volume={14}, ISSN={["1936-086X"]}, DOI={10.1021/acsnano.0c01737}, abstractNote={Solid-state narrow-band light emitters are on-demand for quantum optoelectronics. Current approaches based on defect engineering in low-dimensional materials usually introduce a broad range of emission centers. Here we report narrow-band light emission from covalent heterostructures fused to the edges of graphene nanoribbons (GNRs) by controllable on-surface reactions from molecular precursors. Two types of heterojunction (HJ) states are realized by sequentially synthesizing GNRs and graphene nanodots (GNDs) and then coupling them together. HJs between armchair GNDs and armchair edges of the GNR are coherent and give rise to narrow-band photoluminescence. In contrast, HJs between the armchair GNDs and the zigzag ends of GNRs are defective and give rise to non-radiative states near the Fermi level. At low temperatures, sharp photoluminescence emissions with peak energy range from 2.03 to 2.08 eV and linewidths of 2-5 meV are observed. The radiative HJ states are uniform and the optical transition energy is controlled by the band gaps of GNRs and GNDs. As these HJs can be synthesized in a large quantity with atomic precision, this finding highlights a route to programmable and deterministic creation of quantum light emitters.}, number={4}, journal={ACS NANO}, author={Ma, Chuanxu and Xiao, Zhongcan and Puretzky, Alexander A. and Wang, Hao and Mohsin, Ali and Huang, Jingsong and Liang, Liangbo and Luo, Yingdong and Lawrie, Benjamin J. and Gu, Gong and et al.}, year={2020}, month={Apr}, pages={5090–5098} }
 @article{ma_xiao_huang_liang_lu_hong_sumpter_bernholc_li_2019, title={Direct writing of heterostructures in single atomically precise graphene nanoribbons}, volume={3}, ISSN={2475-9953}, url={http://dx.doi.org/10.1103/PhysRevMaterials.3.016001}, DOI={10.1103/PhysRevMaterials.3.016001}, abstractNote={Precision control of interfacial structures and electronic properties is the key to the realization of functional heterostructures. Here, utilizing the scanning tunneling microscope (STM) both as a manipulation and characterization tool, we demonstrate the fabrication of a heterostructure in a single atomically precise graphene nanoribbon (GNR) and report its electronic properties. The heterostructure is made of a seven-carbon-wide armchair GNR and a lower band gap intermediate ribbon synthesized bottom-up from a molecular precursor on an Au substrate. The short GNR segments are directly written in the ribbon with an STM tip to form atomic precision intraribbon heterostructures. Based on STM studies combined with density functional theory calculations, we show that the heterostructure has a type-I band alignment, with manifestations of quantum confinement and orbital hybridization. Our finding demonstrates a feasible strategy to create a double barrier quantum dot structure with atomic precision for novel functionalities, such as negative differential resistance devices in GNR-based nanoelectronics.}, number={1}, journal={Physical Review Materials}, publisher={American Physical Society (APS)}, author={Ma, Chuanxu and Xiao, Zhongcan and Huang, Jingsong and Liang, Liangbo and Lu, Wenchang and Hong, Kunlun and Sumpter, Bobby G. and Bernholc, J. and Li, An-Ping}, year={2019}, month={Jan} }
 @article{xiao_ma_huang_liang_lu_hong_sumpter_li_bernholc_2018, title={Design of Atomically Precise Nanoscale Negative Differential Resistance Devices}, volume={2}, ISSN={2513-0390 2513-0390}, url={http://dx.doi.org/10.1002/adts.201800172}, DOI={10.1002/adts.201800172}, abstractNote={Downscaling device dimensions to the nanometer range raises significant challenges to traditional device design, due to potential current leakage across nanoscale dimensions and the need to maintain reproducibility while dealing with atomic‐scale components. Here, negative differential resistance (NDR) devices based on atomically precise graphene nanoribbons are investigated. The computational evaluation of the traditional double‐barrier resonant‐tunneling diode NDR structure uncovers important issues at the atomic scale, concerning the need to minimize the tunneling current between the leads while achieving high peak current. A new device structure consisting of multiple short segments that enables high current by the alignment of electronic levels across the segments while enlarging the tunneling distance between the leads is proposed. The proposed structure can be built with atomic precision using a scanning tunneling microscope (STM) tip during an intermediate stage in the synthesis of an armchair nanoribbon. An experimental evaluation of the band alignment at the interfaces and an STM image of the fabricated active part of the device are also presented. This combined theoretical–experimental approach opens a new avenue for the design of nanoscale devices with atomic precision.}, number={2}, journal={Advanced Theory and Simulations}, publisher={Wiley}, author={Xiao, Zhongcan and Ma, Chuanxu and Huang, Jingsong and Liang, Liangbo and Lu, Wenchang and Hong, Kunlun and Sumpter, Bobby G. and Li, An‐Ping and Bernholc, Jerzy}, year={2018}, month={Dec}, pages={1800172} }
 @article{ma_xiao_puretzky_baddorf_lu_hong_bernholc_li_2018, title={Oxidization stability of atomically precise graphene nanoribbons}, volume={2}, ISSN={2475-9953}, url={http://dx.doi.org/10.1103/PhysRevMaterials.2.014006}, DOI={10.1103/physrevmaterials.2.014006}, abstractNote={The stability of graphene nanoribbons (GNRs) against oxidation is critical for their practical applications. Here we study both the thermal stability and the oxidation process of the ambient-exposed armchair GNRs with a width of seven carbon atoms (7-aGNR), grown on an Au(111) surface. The atomic scale evolution of the armchair edges and the zigzag ends of the aGNRs after annealing at different temperatures are revealed by using scanning tunneling microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and first-principles calculations. We observe evidence that the zigzag ends start to be oxidized and decomposed at 180 \ifmmode^\circ\else\textdegree\fi{}C, while the armchair edges are intact at 430 \ifmmode^\circ\else\textdegree\fi{}C but become oxidized at 520 \ifmmode^\circ\else\textdegree\fi{}C. Two different oxygen species are identified at the armchair edges, namely the hydroxyl pair and the epoxy bonding motif with one oxygen bonded to two edge carbons. These oxidization species modify the electronic properties of the pristine 7-aGNRs, with a band-gap reduction from 2.6 to 2.3 eV and 1.9 eV for the hydroxyl pair- and epoxy-terminated edges, respectively. These findings demonstrate the oxidation stability of both the zigzag and armchair edges of GNRs, and they provide an opportunity to harness the high density of edge atoms in applications such as GNR-based high-temperature oxygen sensors.}, number={1}, journal={Physical Review Materials}, publisher={American Physical Society (APS)}, author={Ma, Chuanxu and Xiao, Zhongcan and Puretzky, Alex A. and Baddorf, Arthur P. and Lu, Wenchang and Hong, Kunlun and Bernholc, J. and Li, An-Ping}, year={2018}, month={Jan} }
 @article{ma_liang_xiao_puretzky_hong_lu_meunier_bernholc_li_2017, title={Seamless Staircase Electrical Contact to Semiconducting Graphene Nanoribbons}, volume={17}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/acs.nanolett.7b02938}, DOI={10.1021/acs.nanolett.7b02938}, abstractNote={Electrical contact to low-dimensional (low-D) materials is a key to their electronic applications. Traditional metal contacts to low-D semiconductors typically create gap states that can pin the Fermi level (EF). However, low-D metals possessing a limited density of states at EF can enable gate-tunable work functions and contact barriers. Moreover, a seamless contact with native bonds at the interface, without localized interfacial states, can serve as an optimal electrode. To realize such a seamless contact, one needs to develop atomically precise heterojunctions from the atom up. Here, we demonstrate an all-carbon staircase contact to ultranarrow armchair graphene nanoribbons (aGNRs). The coherent heterostructures of width-variable aGNRs, consisting of 7, 14, 21, and up to 56 carbon atoms across the width, are synthesized by a surface-assisted self-assembly process with a single molecular precursor. The aGNRs exhibit characteristic vibrational modes in Raman spectroscopy. A combined scanning tunneling microscopy and density functional theory study reveals the native covalent-bond nature and quasi-metallic contact characteristics of the interfaces. Our electronic measurements of such seamless GNR staircase constitute a promising first step toward making low resistance contacts.}, number={10}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Ma, Chuanxu and Liang, Liangbo and Xiao, Zhongcan and Puretzky, Alexander A. and Hong, Kunlun and Lu, Wenchang and Meunier, Vincent and Bernholc, J. and Li, An-Ping}, year={2017}, month={Sep}, pages={6241–6247} }