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At least 145 records · Page 8

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DOE OSTI2020

Engineering Edge States of Graphene Nanoribbons for Narrow-Band Photoluminescence

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 nonradiative states near the Fermi level. At low temperatures, sharp photoluminescence emissions with peak energy range from 2.03 to 2.08 eV and line widths of 2–5 meV are observed. Furthermore, 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.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Photo-Induced Charge Transfer of Fullerene and Non-Fullerene Conjugated Polymer Blends via Ab Initio Excited-State Dynamics

Organic conjugated polymers (CPs) are promising candidates for organic photovoltaic (OPV) devices due to their unique tunable mechanical and optoelectronic performance. Over the last decade, optoelectronic properties of narrow band gap CPs as a blend with acceptor units are largely optimized, which leads to noticeable progress in OPV technology. However, their power conversion efficiency is still lower than their organic counterparts (i.e., silicon), limiting their practical usage. In this study, we employ ab initio molecular dynamics to explore photo-induced charge transfer (CT) of the diketopyrrolopyrrole-based polymer as a blend with non-fullerene (i.e., ITIC) and fullerene (i.e., PCBM) acceptor units. The results of charge carrier dynamics induced by selected photoexcitation show that hole density redistribution in space is much faster than electron relaxation. We track the relaxation rates of charge carriers over time, where the derivative of the difference between the rate of electron and hole implies the current density at zero voltage. Furthermore, this can be utilized to characterize the CT performance of CPs blended with different acceptor units. Relaxation rate results indicate that CP blend with ITIC promises a better PV performance, illustrating that the current computational approach opens the door to determine bulk heterojunctions’ electronic performance for OPV devices and narrowing down the list of potential donor–acceptor candidates.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2022

Amphiphilic Peptoid–Directed Assembly of Oligoanilines into Highly Crystalline Conducting Nanotubes

It is reported herein the synthesis of a novel amphiphilic diblock peptoid bearing a terminal conjugated oligoaniline and its self-assembly into small-diameter (D ≈ 35 nm) crystalline nanotubes with high aspect ratios (>30). It is shown that both tetraaniline (TANI)-peptoid and bianiline (BANI)-peptoid triblock molecules self-assemble in solution to form rugged highly crystalline nanotubes that are very stable to protonic acid doping and de-doping processes. The similarity of the crystalline tubular structure of the nanotube assemblies revealed by electron microscopy imaging, and X-ray diffraction analysis of the nanotube assemblies of TANI-functionalized peptoids and nonfunctionalized peptoids showed that the peptoid is an efficient ordered structure directing motif for conjugated oligomers. Films of doped TANI-peptoid nanotubes has a dc conductivity of ca. 95 mS cm –1 , while the thin films of doped un-assembled TANI-peptoids show a factor of 5.6 lower conductivity, demonstrating impact of the favorable crystalline ordering of the assemblies on electrical transport. Furthermore, these results demonstrate that peptoid-directed supramolecular assembly of tethered π-conjugated oligo(aniline) exemplify a novel general strategy for creating rugged ordered and complex nanostructures that have useful electronic and optoelectronic properties.

conducting nanotubes↗

DOE OSTI2022

Robust chain aggregation of low-entropy rigid ladder polymers in solution

Conjugated polymers have been widely investigated where ladder-type conjugated polymers receive more attention due to their rigid backbones and extraordinary properties. However, the understanding of how the rigid conformation of ladder polymers translates to material properties is still limited. Here, we systematically investigated the solution aggregation properties of a carbazole-derived conjugated ladder polymer (LP) and its analogous non-ladder control polymer (CP) via light scattering, neutron scattering, and UV-vis absorption spectroscopy characterization techniques, revealing a highly robust, temperature-insensitive aggregation behavior of the LP. The experimental findings were further validated by computational molecular dynamics simulations. Here we found that the peak positions and intensities of the UV spectra of the LP remained constant between 20 °C and 120 °C in chlorobenzene solution. The polymer also showed a stable hydrodynamic radius measured by dynamic light scattering from 20 °C to 70 °C in the chlorobenzene solution. Using small-angle neutron scattering, no Guinier region was reached in the measured q range down to 0.008 Å –1 , even at elevated temperature. In contrast, the non-ladder control polymer CP was fully soluble in the chlorobenzene solvent without the observation of any notable aggregates. The Brownian dynamics simulation showed that during polymer aggregation, the entropy change of the LP was significantly less negative than that of the non-ladder control polymer. These findings revealed the low entropy nature of rigid conjugated ladder polymers and the low entropy penalty for their aggregation, which is promising for highly robust intermolecular interactions at high temperatures. Such a unique thermodynamic feature of rigid ladder polymers can be leveraged in the design and application of next-generation electronic and optoelectronic devices that function under unconventional high temperature conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Non-Covalent Interactions and Helical Packing in Thiophene-Phenylene Copolymers: Tuning Solid-State Ordering and Charge Transport for Organic Field-Effect Transistors

In this study, we introduce two thiophene-phenylene-thiophene (TPT) polymers designed to leverage noncovalent intramolecular interactions to regulate main-chain conformation and enhance solid-state ordering. By incorporating unsubstituted thiophene (T) or bithiophene (2T) units, we reveal striking divergence in the thermal, morphological, and optoelectronic properties of the resulting films, facilitated by these noncovalent interactions. Using a combination of computational and experimental approaches, we show that annealing yields remarkably different polymer conformations and, consequently, charge transport properties. TPT-T undergoes a significant structural transformation, adopting a more planar backbone conformation and a highly crystalline, edge-on molecular orientation. In contrast, the introduction of a single additional thiophene unit in TPT-2T leads to a more isotropic molecular orientation with a slight preference for face-on alignment, resulting in a heterogeneous film structure that hinders charge transport despite achieving tighter molecular packing. Remarkably, despite being composed of achiral components, TPT-2T develops chirality upon annealing, indicating the formation of a helical conformation. Organic field-effect transistor measurements reveal that the well-ordered alignment in annealed TPT-T films results in higher charge carrier mobility and a narrower distribution of mobility values than in TPT-2T. These findings provide critical insights into the structure−property relationships of conjugated polymers, offering guidance for optimizing molecular design and processing strategies for highperformance organic electronic materials.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Nonfused Nonfullerene Acceptors with an A–D–A'–D–A Framework and a Benzothiadiazole Core for High-Performance Organic Solar Cells

Nonfullerene acceptors (NFAs) have contributed significantly to the progress of organic solar cells (OSCs). However, most NFAs feature a large fused-ring backbone, which usually requires a tedious multiple-step synthesis, and are not applicable to commercial applications. An alternative strategy is to develop nonfused NFAs, which possess synthetic simplicity and facile tunability in optoelectronic properties and solid-state microstructures. In this work, we report two nonfused NFAs, BTCIC and BTCIC-4Cl, based on an A-D-A'-D-A architecture, which possess the same electron-deficient benzothiadiazole central core but different electron-withdrawing terminal groups. The optical properties, energy levels, and molecular crystallinities were finely tuned by changing the terminal groups. Moreover, a decent power conversion efficiency of 9.3 and 10.5% has been achieved by BTCIC and BTCIC-4Cl, respectively, by blending them with an appropriate polymer donor. These results demonstrate the potential of A-D-A'-D-A type nonfused NFAs for high-performance OSCs. Further development of nonfused NFAs will be very fruitful by employing appropriate building blocks and via side-chain optimizations.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Strong Induced Circular Dichroism in a Hybrid Lead‐Halide Semiconductor Using Chiral Amino Acids for Crystallite Surface Functionalization

Abstract Chirality is a desired property in functional semiconductors for optoelectronic, catalytic, and spintronic applications. Here, introducing enantiomerically‐pure 3‐aminobutyric acid (3‐ABA) into thin films of the 1D semiconductor dimethylammonium lead iodide (DMAPbI 3 ) is found to result in strong circular dichroism (CD) in the optical absorption. X‐ray diffraction and grazing incidence small angle X‐ray scattering (GISAXS) are applied to gain molecular‐scale insights into the chirality transfer mechanism, which is attributed to a chiral surface modification of DMAPbI 3 crystallites. This study demonstrates that the CD signal strength can be controlled by the amino‐acid content relative to the crystallite surface area. The CD intensity is tuned by the composition of the precursor solution and the spin‐coating time, thereby achieving anisotropy factors ( g abs ) as high as 1.75 × 10 –2 . Grazing incidence wide angle scattering reveals strong preferential ordering that can be suppressed via tailored synthesis conditions. Different contributions to the chiroptical properties are resolved by a detailed analysis of the CD signal utilizing an approach based on the Mueller matrix model. This report of a novel class of chiral hybrid semiconductors with precise control over their optical activity presents a promising approach for the design of circularly polarized light detectors and emitters.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Heterocyclic Chromophore Amphiphiles and their Supramolecular Polymerization

Abstract Supramolecular polymerization of π‐conjugated amphiphiles in water is an attractive approach to create functional nanostructures. Here, we report on the synthesis, optoelectronic and electrochemical properties, aqueous supramolecular polymerization, and conductivity of polycyclic aromatic dicarboximide amphiphiles. The chemical structure of the model perylene monoimide amphiphile was modified with heterocycles, essentially substituting one fused benzene ring with thiophene, pyridine or pyrrole rings. All the heterocycle‐containing monomers investigated underwent supramolecular polymerization in water. Large changes to the monomeric molecular dipole moments led to nanostructures with low electrical conductivity due to diminished interactions. Although the substitution of benzene with thiophene did not notably change the monomer dipole moment, it led to crystalline nanoribbons with 20‐fold higher electrical conductivity, due to enhanced dispersion interactions as a result of the presence of sulfur atoms.

Đorđević, Luka↗

DOE OSTI2023

Heterocyclic Chromophore Amphiphiles and their Supramolecular Polymerization

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Lattice thermal transport in two-dimensional alloys and fractal heterostructures

Abstract Engineering thermal transport in two dimensional materials, alloys and heterostructures is critical for the design of next-generation flexible optoelectronic and energy harvesting devices. Direct experimental characterization of lattice thermal conductivity in these ultra-thin systems is challenging and the impact of dopant atoms and hetero-phase interfaces, introduced unintentionally during synthesis or as part of deliberate material design, on thermal transport properties is not understood. Here, we use non-equilibrium molecular dynamics simulations to calculate lattice thermal conductivity of $${\mathrm {(Mo|W)Se_2}}$$ ( Mo | W ) Se 2 monolayer crystals including $${\mathrm {Mo}}_{1-x}{\mathrm {W}}_x{\mathrm {Se_2}}$$ Mo 1 - x W x Se 2 alloys with substitutional point defects, periodic $${\mathrm {MoSe_2}|\mathrm {WSe_2}}$$ MoSe 2 | WSe 2 heterostructures with characteristic length scales and scale-free fractal $${\mathrm {MoSe_2}}|{\mathrm {WSe_2}}$$ MoSe 2 | WSe 2 heterostructures. Each of these features has a distinct effect on phonon propagation in the crystal, which can be used to design fractal and periodic alloy structures with highly tunable thermal conductivities. This control over lattice thermal conductivity will enable applications ranging from thermal barriers to thermoelectrics.

42 ENGINEERING↗

DOE OSTI2025

Polaron Delocalization and Transport in Doped Graphene Nanoribbon Thin Films

Graphene nanoribbons (GNRs) are quantum-confined π-conjugated monolayer semiconductors with attractive properties for optoelectronic applications. However, the ground- and excited-state properties of charge carriers in GNRs are still poorly understood, particularly with regards to the coupling between charges and the GNR lattice and the degree to which this coupling impacts local and macroscopic charge transport. To address this issue, we systematically correlate carrier density-dependent charge transport with spectroscopic modulations in chemically doped thin films of armchair graphene nanoribbons (9-aGNRs). This study combines Fourier transform infrared (FTIR) and ultraviolet−visible−near-infrared (UV−vis−NIR) spectroscopy with both local and macroscopic conductivity measurements to arrive at a full and self-consistent picture of transport in doped GNR thin films. Using three different molecular p-type dopants (i.e., oxidants), we demonstrate that hole polarons are the dominant quasi-particle determining charge transport in GNRs and that the degree of polaron delocalization depends sensitively on the dopant and the hole density. For all three dopants, the local conductivity probed by microwave spectroscopy substantially exceeds the long-range conductivity obtained by four-point probe measurements. Interestingly, the dopant size substantially influences charge transport at high hole densities. We ascribe this effect to different propensities for forming bipolarons with lower mobilities than polarons. Comparison of GNR transport and spectral properties to other prototypical π-conjugated semiconductors (e.g., semiconducting polymers or carbon nanotubes) benchmark the charge transport properties of GNR thin films for optoelectronic devices and applications.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2025

When Pore Met Semi: Charting the Rise of Porous Metal Halide Semiconductors

Metal halide semiconductors (MHS) are a versatile class of materials with fully customizable mechanical and optoelectronic properties that have proven to be prominent candidates in numerous impactful applications. Finding a way to generate porosity in MHS would bestow upon them an additional node in property engineering, thus allowing them to be utilized in uncharted technologies. Motivated by this promise, we developed a general strategy to render the MHS porous. We employed molecular cages as structure-directing agents and countercations, which fostered a new family of materials: porous metal halide semiconductors (PMHS). The presence of molecular cages gave rise to ultramicroporous structures imposed by the organic part cavities and a record water stability performance of 27 months so far. In this Perspective, we discuss the principles and promises of PMHS, which combine the merits of porous and electronic compounds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2026

Hydrogen-Bonded Organic Framework Enables Phase-Pure Layered Tin Perovskite Nanowires for Room-Temperature Lasing

Room-temperature lasing is a key milestone in the development of miniaturized optoelectronic and photonic devices. We present a simple approach to synthesize phase-pure quasi-2D layered tin perovskite nanowires with varying quantum well thicknesses (n = 1 to 4). By incorporating a new organic spacer capable of forming a hydrogen-bonded organic framework, this method promoted anisotropic crystal growth and enhanced lattice rigidity. Furthermore, introducing molecular intercalants enabled controlled crystallization into well-defined nanowires that function as Fabry−Pérot cavities. Cavities made from n = 2 to 4 perovskites support efficient and robust nearinfrared, room-temperature optically pumped lasing with the threshold as low as 75.8 μJ/cm 2 , cavity quality factor over 3000, and negligible degradation over 106 pulses. A cleaved coupled nanolaser was fabricated as a proof-of-concept device for photonic applications.

Cavities↗

DOE OSTI2023

Comparing the structures and photophysical properties of two charge transfer co-crystals

Organic co-crystals have emerged as a promising class of semiconductors for next-generation optoelectronic devices due to their unique photophysical properties. This paper presents a joint experimental-theoretical study comparing the crystal structure, spectroscopy, and electronic structure of two charge transfer co-crystals. Reported herein is a novel co-crystal Npe:TCNQ, formed from 4-(1-naphthylvinyl)pyridine (Npe) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) via molecular self-assembly. This work also presents a revised study of the co-crystal composed of Npe and 1,2,4,5-tetracyanobenzene (TCNB) molecules, Npe:TCNB, herein reported with a higher-symmetry (monoclinic) crystal structure than previously published. Npe:TCNB and Npe:TCNQ dimer clusters are used as theoretical model systems for the co-crystals; the geometries of the dimers are compared to geometries of the extended solids, which are computed with periodic boundary conditions density functional theory. UV-Vis absorption spectra of the dimers are computed with time-dependent density functional theory and compared to experimental UV-Vis diffuse reflectance spectra. Both Npe:TCNB and Npe:TCNQ are found to exhibit neutral character in the S 0 state and ionic character in the S 1 state. In conclusion, the high degree of charge transfer in the S 1 state of both Npe:TCNB and Npe:TCNQ is rationalized by analyzing the changes in orbital localization associated with the S 1 transitions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Dynamic metal-linker bonds in metal–organic frameworks

Metal-linker bonds serve as the “glue” that binds metal ions to multitopic organic ligands in the porous materials known as metal–organic frameworks (MOFs). Despite ample evidence of bond lability in molecular and polymeric coordination compounds, the metal-linker bonds of MOFs were long assumed to be rigid and static. Given the importance of ligand fields in determining the behaviour of metal species, labile bonding in MOFs would help explain outstanding questions about MOF behaviour, while providing a design tool for controlling dynamic and stimuli-responsive optoelectronic, magnetic, catalytic, and mechanical phenomena. Here, in this work, we present emerging evidence that MOF metal-linker bonds exist in dynamic equilibria between weakly and tightly bond conformations, and that these equilibria respond to guest–host chemistry, drive phase change behavior, and exhibit size-dependence in MOF nanoparticles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Synthesis of TiO{sub 2} nanoparticles decorated with gold nanoclusters using pulsed laser ablation in liquid

Amorphous spherical titanium dioxide (TiO{sub 2}) nanoparticles, decorated gold nanoparticles, are shown to be formed under irradiation of commercial TiO{sub 2} nanopowders dispersed in aqueous solution of hydrogen tetrachloroaurate (HAuCl{sub 4}) by second-harmonic nanosecond pulses of an Nd : YAG laser. It is found that these hybrid nanostructures are formed during laser-induced remelting of the initial nanoparticles, stimulated by gold nanoclusters (providing more efficient absorption of visible laser radiation by titanium dioxide) recovered on their surface. The morphology and chemical composition of the newly formed hybrid nanomaterials are investigated in detail by electron microscopy, Raman spectroscopy, and energy-dispersive analysis. It is shown that the average size and number of gold nanoparticles recovered on the TiO{sub 2} surface can be controlled by varying the concentration ratio of the initial nanomaterial and HAuCl{sub 4}. A spectroscopic analysis of light scattering by single hybrid nanoparticles and the results of numerical calculation of the structure of electromagnetic fields near their surface indicate good prospects of these hybrid nanomaterials for various applications in modern optics, optoelectronics, and nanophotonics; for example, for designing chemo- and biosensor platforms and new-generation solar cells. (nanoparticles)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2024

Defect Self-Elimination in Nanocube Superlattices Through the Interplay of Brownian, van der Waals, and Ligand-Based Forces and Torques

Understanding defect healing is necessary for realizing devices based on nanoparticle-superlattices with controlled electronic and optoelectronic performance. However, key questions remain regarding nanoparticle interactions and resulting assembly dynamics and defect self-elimination. In particular, for anisotropic particles, additional degrees of freedom beyond those of spherical particles, such as rotational dynamics and toques, significantly impact phenomena. Here, in this work, we investigate nanocube (NC) superlattices by employing liquid phase transmission electron microscopy, continuum theories and molecular dynamics simulations. Analyzing interparticle forces and torques due to van der Waals, Brownian, and ligand interactions, we find that the latter dominates and that the anisotropic NC morphology introduces significant torques. In imperfect regions, unbalanced forces and torques induce NC translations and rotations that are transmitted to neighboring NCs, prompting “chain interactions” in a 2D network, which lead to defect self-elimination. This fundamental understanding will further enable design and fabrication of defect-free superlattices, as well as those with tailored defects, via assembly of anisotropic particles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Tailoring Interface Energies via Phosphonic Acids to Grow and Stabilize Cubic FAPbI 3 Deposited by Thermal Evaporation

Coevaporation of formamidinium lead iodide (FAPbI 3 ) is a promising route for the fabrication of highly efficient and scalable optoelectronic devices, such as perovskite solar cells. However, it poses experimental challenges in achieving stoichiometric FAPbI 3 films with a cubic structure (α-FAPbI 3 ). In this work, we show that undesired hexagonal phases of both PbI 2 and FAPbI 3 form during thermal evaporation, including the well-known 2H-FAPbI 3 , which are detrimental for optoelectronic performance. We demonstrate the growth of α-FAPbI 3 at room temperature via thermal evaporation by depositing phosphonic acids (PAc) on substrates and subsequently coevaporating PbI 2 and formamidinium iodide. We use density-functional theory to develop a theoretical model to understand the relative growth energetics of the α and 2H phases of FAPbI 3 for different molecular interactions. Experiments and theory show that the presence of PAc molecules stabilizes the formation of α-FAPbI 3 in thin films when excess molecules are available to migrate during growth. This migration of molecules facilitates the continued presence of adsorbed organic precursors at the free surface throughout the evaporation, which lowers the growth energy of the α-FAPbI 3 phase. Our theoretical analyses of PAc molecule–molecule interactions show that ligands can form hydrogen bonding to reduce the migration rate of the molecules through the deposited film, limiting the effects on the crystal structure stabilization. Our results also show that the phase stabilization with molecules that migrate is long-lasting and resistant to moist air. These findings enable reliable formation and processing of α-FAPbI 3 films via vapor deposition.

36 MATERIALS SCIENCE↗