Structure Formation and Coupling Reactions of Hexaphenylbenzene and Its Brominated Analog

The 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

Structure Formation and Coupling Reactions of Hexaphenylbenzene and Its Brominated Analog

The 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

Structure Formation and Coupling Reactions of Hexaphenylbenzene and Its Brominated Analog

The on‐surface coupling of the prototypical precursor molecule for graphene nanoribbon synthesis, 6,11‐dibromo‐1,2,3,4‐tetraphenyltriphenylene (C(42)Br(2)H(26), TPTP), and its non‐brominated analog hexaphenylbenzene (C(42)H(30), 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

On-Surface Characterization of Atomically Precise Graphene Nanoribbons: Variations in Structure, Substrate, and Deposition Method

Graphene-based materials have been the focus of intense research in recent years, and owing to the desirable properties they possess, the variety of potential applications for them is likewise impressive. Chapter 1 discusses graphene nanoribbons (GNRs) and how their precise atomic features exhibit critical influence over electronic characteristics such as band gaps, magnetic edge states, and topological phases. One can therefore engage in design of molecular precursors to build atomically precise GNRs from the bottom up with desired features which can be probed with surface science techniques. These elements include ribbon width, edge geometry, heteroatomic doping, and many more. In this dissertation are results of scanning probe microscopy experiments to examine structural and electronic characteristics of several GNR structures derived from surface-assisted synthesis techniques. First, we studied the growth of chevron GNRs on Cu(111) (Chapter 2). Epitaxial growth of chevron polymers and GNRs at low temperatures arose out of the strong interactions between structure and surface. In Chapter 3 we discuss the development of an alternative deposition technique, direct contact transfer (DCT). It arose as an analogue of dry contact transfer for solution-synthesized GNRs. The procedure was demonstrated for two precursors, and the intermediates observed in the process of annealing were determined by DFT to be stabilized by π-π interactions. Chapter 4 covers the on-surface synthesis of laterally extended chevron GNRs (eGNRs). By slight modification of the chevron precursor structure, the band gap of the GNRs was reduced in accordance with established theory. In Chapter 5 results are presented on the on-surface synthesis of intrinsically porous chevron-derived GNRs. These GNRs featured electronic states at their pores as well as reduced band gaps relative to chevron GNRs. Chapter 6 details work on the growth of GNRs from a precursor with two centers of rotation and the potential for influencing its self-assembly. Finally, Chapter 7 contains a summary of the work performed as well as directions to be explored in the future as relate to this project

On-Surface Characterization of Atomically Precise Graphene Nanoribbons: Variations in Structure, Substrate, and Deposition Method

Graphene-based materials have been the focus of intense research in recent years, and owing to the desirable properties they possess, the variety of potential applications for them is likewise impressive. Chapter 1 discusses graphene nanoribbons (GNRs) and how their precise atomic features exhibit critical influence over electronic characteristics such as band gaps, magnetic edge states, and topological phases. One can therefore engage in design of molecular precursors to build atomically precise GNRs from the bottom up with desired features which can be probed with surface science techniques. These elements include ribbon width, edge geometry, heteroatomic doping, and many more. In this dissertation are results of scanning probe microscopy experiments to examine structural and electronic characteristics of several GNR structures derived from surface-assisted synthesis techniques. First, we studied the growth of chevron GNRs on Cu(111) (Chapter 2). Epitaxial growth of chevron polymers and GNRs at low temperatures arose out of the strong interactions between structure and surface. In Chapter 3 we discuss the development of an alternative deposition technique, direct contact transfer (DCT). It arose as an analogue of dry contact transfer for solution-synthesized GNRs. The procedure was demonstrated for two precursors, and the intermediates observed in the process of annealing were determined by DFT to be stabilized by π-π interactions. Chapter 4 covers the on-surface synthesis of laterally extended chevron GNRs (eGNRs). By slight modification of the chevron precursor structure, the band gap of the GNRs was reduced in accordance with established theory. In Chapter 5 results are presented on the on-surface synthesis of intrinsically porous chevron-derived GNRs. These GNRs featured electronic states at their pores as well as reduced band gaps relative to chevron GNRs. Chapter 6 details work on the growth of GNRs from a precursor with two centers of rotation and the potential for influencing its self-assembly. Finally, Chapter 7 contains a summary of the work performed as well as directions to be explored in the future as relate to this project

Few-layer titanium trisulfide (TiS\u3csub\u3e3\u3c/sub\u3e) field-effect transistors

Titanium trisulfide (TiS3) is a promising layered semiconductor material. Several-mm-long TiS3 whiskers can be conveniently grown by the direct reaction of titanium and sulfur. In this study, we exfoliated these whiskers using the adhesive tape approach and fabricated few-layer TiS3 field-effect transistors (FETs). The TiS3 FETs showed an n-type electronic transport with roomtemperature field-effect mobilities of 18-24 cm2V-1s-1 and ON/OFF ratios up to 300. We demonstrate that TiS3 is compatible with the conventional atomic layer deposition (ALD) procedure for Al2O3. ALD of alumina on TiS3 FETs resulted in mobility increase up to 43 cm2V-1s-1, ON/OFF ratios up to 7000, and much improved subthreshold swing characteristics. This study shows that TiS3 is a competitive electronic material in the family of two-dimensional (2D) transition metal chalcogenides and can be considered for emerging device applications

Structure and proton-transfer mechanism in one-dimensional chains of benzimidazoles

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 π-electron system, of the type −CN···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

Radiostereometric Analysis Permits in Vivo Measurement of Very Small Levels of Wear in TKA

© 2019 by the Association of Bone and Joint Surgeons. Unauthorized reproduction of this article is prohibited. Background Revision of TKA as a result of polyethylene wear is decreasing, but long-term wear performance of polyethylene is still a topic of interest to surgeons and device manufacturers seeking to improve longevity. Measuring wear of modern, wear-resistant implants has been described using radiostereometric analysis (RSA). Performing in vivo measurements would establish whether implant retrieval studies are representative of wear in well-performing knees.Questions/purposesFor a single knee implant system, we sought to determine (1) the linear wear rate using RSA; (2) the association between demographic factors and wear rate; and (3) the association between limb alignment and wear rate.MethodsA total of 49 patients with a minimum followup of 10 years (median, 12 years; range, 10-20 years) were retrospectively selected. During the examined period, 4082 TKAs were performed of which 2085 were the implant examined in this study. There were 71 of these patients who met the criteria including an available full-leg radiograph postoperatively, and 34 of these patients returned for examination along with 15 additional from a separate RSA study that also met the criteria. All patients received a posterior-stabilized, cobalt-chromium-on-conventional polyethylene total knee implant from a single implant system, which was the most commonly used at our institution at the time. Patients underwent standing RSA examinations from 0° to 120° of flexion at a single time point without the use of marker beads. Linear wear rates (including creep) were measured based on intersections between the femoral component and tibial insert models. Associations between wear and patient age at surgery, sex, height, weight, body mass index, tibial insert size, and limb alignment were examined.ResultsUsing the maximum linear wear rate from any flexion angle, the lateral rate was 0.047 mm/year (interquartile range [IQR], 0.034-0.066 mm/year) and the medial rate was 0.052 mm/year (IQR, 0.040-0.069 mm/year). Using the median of the linear wear rates across all flexion angles, the lateral rate was 0.027 mm/year (IQR, 0.017-0.046 mm/year) and the medial rate was 0.038 mm/year (IQR, 0.022-0.054 mm/year). This rate for males was 0.049 mm/year medially (IQR, 0.042-0.077 mm/year) and 0.032 mm/year laterally (IQR, 0.026-0.059 mm/year), and for females was 0.027 mm/year medially (0.016-0.039 mm/year) and 0.020 mm/year laterally (IQR, 0.013-0.032 mm/year). The wear rate for males was greater medially (difference = 0.022 mm/year, p \u3c 0.001) and laterally (difference = 0.012 mm/year, p = 0.008). There were associations between greater wear and increasing height (ρ = 0.48, p \u3c 0.001 medially and ρ = 0.30, p = 0.04 laterally), decreasing body mass index (ρ = -0.31, p = 0.03 medially), and greater implant size (ρ = 0.34, p = 0.02 medially). Increasingly varus leg alignment was associated with greater medial wear (ρ = 0.33, p = 0.02).ConclusionsGreater wear rates were associated with demographic factors and leg alignment. Further RSA wear studies of other modern implant systems would provide complementary information to retrieval studies and valuable data on wear resistance.Clinical RelevanceGood wear resistance was demonstrated by well-performing implants in patients at long-term followup with wear magnitudes in agreement with reported values from retrieval studies

Local manifestations of thickness-dependent topology and edge states in the topological magnet MnBi 2 Te 4

The interplay of nontrivial 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 MnBi2Te4 (MBT), using scanning tunneling microscopy and spectroscopy on molecular beam epitaxy grown MBT thin films. A thickness-dependent band gap 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 odd and even layers, respectively. At step edges, we observe localized electronic states, in general agreement with axion insulator and QAH edge states, respectively, indicating topological phase transitions across the steps. 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 application