Tailoring on-surface molecular reactions and assembly through hydrogen-modified synthesis: From triarylamine monomer to 2D covalent organic framework

Relative to conventional wet-chemical synthesis techniques, on-surface synthesis of organic networks in ultrahigh vacuum has few control parameters. The molecular deposition rate and substrate temperature are typically the only synthesis variables to be adjusted dynamically. Here we demonstrate that reducing conditions in the vacuum environment can be created and controlled without dedicated sources -- relying only on backfilled hydrogen gas and ion gauge filaments -- and can dramatically influence the Ullmann-like on-surface reaction used for synthesizing two-dimensional covalent organic frameworks (2D COFs). Using tribromo dimethylmethylene-bridged triphenylamine ((Br$_3$)DTPA) as monomer precursors, we find that atomic hydrogen blocks aryl-aryl bond formation. Control of the relative monomer and hydrogen fluxes is used to produce large islands of self-assembled monomers, dimers, or macrocycle hexamers. On-surface synthesis of these oligomers, from a single precursor, circumvents potential challenges with protracted wet-chemical synthesis or low precursor volatility for large molecules. Using scanning tunneling microscopy and spectroscopy (STM/STS), we show that changes in the electronic states through this oligomer sequence provide an insightful view of the 2D-COF (synthesized in the absence of atomic hydrogen) as the endpoint in an evolution of electronic structures from the monomer

Chargeâ Transport Properties of F6TNAPâ Based Chargeâ Transfer Cocrystals

The crystal structures of the chargeâ transfer (CT) cocrystals formed by the Ï â electron acceptor 1,3,4,5,7,8â hexafluoroâ 11,11,12,12â tetracyanonaphthoâ 2,6â quinodimethane (F6TNAP) with the planar Ï â electronâ donor molecules triphenylene (TP), benzo[b]benzo[4,5]thieno[2,3â d]thiophene (BTBT), benzo[1,2â b:4,5â bâ ²]dithiophene (BDT), pyrene (PY), anthracene (ANT), and carbazole (CBZ) have been determined using singleâ crystal Xâ ray diffraction (SCXRD), along with those of two polymorphs of F6TNAP. All six cocrystals exhibit 1:1 donor/acceptor stoichiometry and adopt mixedâ stacking motifs. Cocrystals based on BTBT and CBZ Ï â electron donor molecules exhibit brickwork packing, while the other four CT cocrystals show herringboneâ type crystal packing. Infrared spectroscopy, molecular geometries determined by SCXRD, and electronic structure calculations indicate that the extent of groundâ state CT in each cocrystal is small. Density functional theory calculations predict large conduction bandwidths and, consequently, low effective masses for electrons for all six CT cocrystals, while the TPâ , BDTâ , and PYâ based cocrystals are also predicted to have large valence bandwidths and low effective masses for holes. Chargeâ carrier mobility values are obtained from spaceâ charge limited current (SCLC) measurements and fieldâ effect transistor measurements, with values exceeding 1 cm2 Vâ 1 s1 being estimated from SCLC measurements for BTBT:F6TNAP and CBZ:F6TNAP cocrystals.Structural, electronic band structure, and electrical properties of a series of chargeâ transfer cocrystals based on F6TNAP and six planar donors are presented. Density functional theory calculations afford large conduction bandwidths and low effective masses for all six cocrystals. A few cocrystals exhibit chargeâ carrier mobilities in excess of 1 cm2 Vâ 1 sâ 1, as estimated from spaceâ charge limited current measurements.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153248/1/adfm201904858-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153248/2/adfm201904858.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153248/3/adfm201904858_am.pd

Design and Synthesis of Two-Dimensional Covalent Organic Frameworks with Four-Arm Cores: Prediction of Remarkable Ambipolar Charge-Transport Properties

We have considered three two-dimensional (2D) π-conjugated polymer networks (i.e., covalent organic frameworks, COFs) materials based on pyrene, porphyrin, and zinc-porphyrin cores connected via diacetylenic linkers. Their electronic structures, investigated at the density functional theory global-hybrid level, are indicative of valence and conduction bands that have large widths, ranging between 1 and 2 eV. Using a molecular approach to derive the electronic couplings between adjacent core units and the electron-vibration couplings, the three π-conjugated 2D COFs are predicted to have ambipolar charge-transport characteristics with electron and hole mobilities in the range of 65-95 cm2V-1s-1. Such predicted values rank these 2D COFs among the highest-mobility organic semiconductors. In addition, we have synthesized the zinc-porphyrin based 2D COF and carried out structural characterization via powder X-ray diffraction and surface area analysis, which demonstrates the feasability of these electroactive networks.</p

Thermo-cross-linkable fullerene for long-term stability of photovoltaic devices

Oligomeric fullerene formedin situfrom a thermo-cross-linkable fullerene molecule provides better morphology control and long term device stability for bulk heterojunction based organic photovoltaics.</p

Ultra-low p-doping of poly(3-hexylthiophene) and its impact on polymer aggregation and photovoltaic performance

Poly(3-hexylthiophene) (P3HT) films and P3HT / fullerene photovoltaic cells have been p-doped with very low levels (< 1 wt. %) of molybdenum tris[1-(trifluoromethylcarbonyl)- 2-(trifluoromethyl)-ethane-1,2-dithiolene]. The dopants are inhomogenously distributed within doped P3HT films, both laterally and as a function of depth, and appear to aggregate in some instances. Doping also results in subtle changes in the local and long range order of the P3HT film. These effects likely contribute to the complexity of the observed evolutions in conductivity, mobility and work function with doping levels. They also negatively affect the open-circuit voltage and fill factor of solar cells in unexpected ways, indicating that dopant aggregation and non-uniform distribution can harm device performance

Short and Long-Range Electron Transfer Compete to Determine Free-Charge Yield in Organic Semiconductors

Understanding how Frenkel excitons efficiently split to form free-charges in low-dielectric constant organic semiconductors has proven challenging, with many different models proposed in recent years to explain this phenomenon. Here, we present evidence that a simple model invoking a modest amount of charge delocalization, a sum over the available microstates, and the Marcus rate constant for electron transfer can explain many seemingly contradictory phenomena reported in the literature. We use an electron-accepting fullerene host matrix dilutely sensitized with a series of electron donor molecules to test this hypothesis. The donor series enables us to tune the driving force for photoinduced electron transfer over a range of 0.7 eV, mapping out normal, optimal, and inverted regimes for free-charge generation efficiency, as measured by time-resolved microwave conductivity. However, the photoluminescence of the donor is rapidly quenched as the driving force increases, with no evidence for inverted behavior, nor the linear relationship between photoluminescence quenching and charge-generation efficiency one would expect in the absence of additional competing loss pathways. This behavior is self-consistently explained by competitive formation of bound charge-transfer states and long-range or delocalized free-charge states, where both rate constants are described by the Marcus rate equation. Moreover, the model predicts a suppression of the inverted regime for high-concentration blends and efficient ultrafast free-charge generation, providing a mechanistic explanation for why Marcus-inverted-behavior is rarely observed in device studies

Charge‐Transport Properties of F 6

The crystal structures of the chargeâ transfer (CT) cocrystals formed by the Ï â electron acceptor 1,3,4,5,7,8â hexafluoroâ 11,11,12,12â tetracyanonaphthoâ 2,6â quinodimethane (F6TNAP) with the planar Ï â electronâ donor molecules triphenylene (TP), benzo[b]benzo[4,5]thieno[2,3â d]thiophene (BTBT), benzo[1,2â b:4,5â bâ ²]dithiophene (BDT), pyrene (PY), anthracene (ANT), and carbazole (CBZ) have been determined using singleâ crystal Xâ ray diffraction (SCXRD), along with those of two polymorphs of F6TNAP. All six cocrystals exhibit 1:1 donor/acceptor stoichiometry and adopt mixedâ stacking motifs. Cocrystals based on BTBT and CBZ Ï â electron donor molecules exhibit brickwork packing, while the other four CT cocrystals show herringboneâ type crystal packing. Infrared spectroscopy, molecular geometries determined by SCXRD, and electronic structure calculations indicate that the extent of groundâ state CT in each cocrystal is small. Density functional theory calculations predict large conduction bandwidths and, consequently, low effective masses for electrons for all six CT cocrystals, while the TPâ , BDTâ , and PYâ based cocrystals are also predicted to have large valence bandwidths and low effective masses for holes. Chargeâ carrier mobility values are obtained from spaceâ charge limited current (SCLC) measurements and fieldâ effect transistor measurements, with values exceeding 1 cm2 Vâ 1 s1 being estimated from SCLC measurements for BTBT:F6TNAP and CBZ:F6TNAP cocrystals.Structural, electronic band structure, and electrical properties of a series of chargeâ transfer cocrystals based on F6TNAP and six planar donors are presented. Density functional theory calculations afford large conduction bandwidths and low effective masses for all six cocrystals. A few cocrystals exhibit chargeâ carrier mobilities in excess of 1 cm2 Vâ 1 sâ 1, as estimated from spaceâ charge limited current measurements.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/153248/1/adfm201904858-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153248/2/adfm201904858.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/153248/3/adfm201904858_am.pd

Solution-Processed Doping of Trilayer WSe₂ with Redox-Active Molecules

The development of processes to controllably dope two-dimensional semiconductors is critical to achieving next-generation electronic and optoelectronic devices. In this study, n- and p-doping of highly uniform large-area trilayer WSe₂ is achieved by treatment with solutions of molecular reductants and oxidants. The sign and extent of doping can be conveniently controlled by the redox potential of the (metal−)­organic molecules, the concentration of dopant solutions, and the treatment time. Threshold voltage shifts, the direction of which depends on whether a p- or n-dopant is used, and tunable channel current are observed in doped WSe₂ field-effect transistors. Detailed physical characterization including photoemission (ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy) and Raman spectroscopy provides fundamental understanding of the underlying mechanism. The origin of the doping is the electron-transfer reactions between molecular dopants and 2D semiconductors and results in a shift of the Fermi level relative to the valence band due both to state filling/emptying and to large surface dipoles between the dopant ions and the oppositely charged WSe₂. These two effects both contribute to large work function changes of up to ±1 eV

Electronically Coupled 2D Polymer/MoS2 Heterostructures.

Emergent quantum phenomena in electronically coupled two-dimensional heterostructures are central to next-generation optical, electronic, and quantum information applications. Tailoring electronic band gaps in coupled heterostructures would permit control of such phenomena and is the subject of significant research interest. Two-dimensional polymers (2DPs) offer a compelling route to tailored band structures through the selection of molecular constituents. However, despite the promise of synthetic flexibility and electronic design, fabrication of 2DPs that form electronically coupled 2D heterostructures remains an outstanding challenge. Here, we report the rational design and optimized synthesis of electronically coupled semiconducting 2DP/2D transition metal dichalcogenide van der Waals heterostructures, demonstrate direct exfoliation of the highly crystalline and oriented 2DP films down to a few nanometers, and present the first thickness-dependent study of 2DP/MoS2 heterostructures. Control over the 2DP layers reveals enhancement of the 2DP photoluminescence by two orders of magnitude in ultrathin sheets and an unexpected thickness-dependent modulation of the ultrafast excited state dynamics in the 2DP/MoS2 heterostructure. These results provide fundamental insight into the electronic structure of 2DPs and present a route to tune emergent quantum phenomena in 2DP hybrid van der Waals heterostructures