DJL

Structurally well-defined graphene nanoribbons (GNRs) have attracted great interest because of their unique optical, electronic, and magnetic properties. However, strong π–π interactions within GNRs result in poor liquid-phase dispersibility, which impedes further investigation of these materials in numerous research areas, including supramolecular self-assembly. Structurally defined GNRs were synthesized by a bottom-up strategy, involving grafting of hydrophilic poly(ethylene oxide) (PEO) chains of different lengths (GNR-PEO). PEO grafting of 42–51 % percent produces GNR-PEO materials with excellent dispersibility in water with high GNR concentrations of up to 0.5 mg mL⁻¹. The “rod–coil” brush-like architecture of GNR-PEO resulted in 1D hierarchical self-assembly behavior in the aqueous phase, leading to the formation of ultralong nanobelts, or spring-like helices, with tunable mean diameters and pitches. In aqueous dispersions the superstructures absorbed in the near-infrared range, which enabled highly efficient conversion of photon energy into thermal energy.

Supramolecular nanostructures of structurally well-defined graphene nanoribbons grafted with hydrophilic poly(ethylene oxide) chains present excellent dispersibility in the aqueous phase. Aqueous dispersions of graphene nanoribbon superstructures absorb in the near-infrared range, thereby enabling highly efficient conversion of photon energy into thermal energy.

Lead-halide perovskites are well known to decompose rapidly when exposed to polar solvents, such as water. Contrary to this common-place observation, we have found that through introducing a suitable minor amount of water into the reaction mixture, we can synthesize stable CsPbBr₃ nanocrystals. The size and the crystallinity, and as a result the band gap tunability of the strongly emitting CsPbBr₃ nanocrystals correlate with the water content. Suitable amounts of water change the crystallization environment, inducing the formation of differently shaped perovskites, namely spherical NCs, rectangular nanoplatelets, or nanowires. Bright CsPbBr₃ nanocrystals with the photoluminescence quantum yield reaching 90 % were employed for fabrication of inverted hybrid inorganic/organic light-emitting devices, with the peak luminance of 4428 cd m⁻² and external quantum yield of 1.7 %.

Lead-halide perovskites are well known to decompose rapidly when exposed to polar solvents. Contrary to this, stable CsPbBr₃ nanocrystals could be synthesized through introducing a suitable minor amount of water into the reaction mixture. The size and the crystallinity, and as a result the band gap tunability of the strongly emitting CsPbBr₃ nanocrystals correlate with the water content.

Organic–inorganic perovskites have made tremendous progress in recent years due to exceptional material properties such as high panchromatic absorption, charge carrier diffusion lengths, and a sharp optical band edge. The combination of high-quality semiconductor performance with low-cost deposition techniques seems to be a match made in heaven, creating great excitement far beyond academic ivory towers. This is particularly true for perovskite solar cells (PSCs) that have shown unprecedented gains in efficiency and stability over a time span of just five years. Now there are serious efforts for commercialization with the hope that PSCs can make a major impact in generating inexpensive, sustainable solar electricity. In this Review, we will focus on perovskite material properties as well as on devices from the atomic to the thin film level to highlight the remaining challenges and to anticipate the future developments of PSCs.

Perovskite solar cells have emerged as a low-cost, thin-film technology with unprecedented efficiency gains that challenge the quasi-paradigm that high efficiency photovoltaics must come at high costs. Perovskites can be processed via inexpensive solution methods and have exceptional material properties that are comparable to established materials such as CdTe, GaAs, or Si. Remarkably, perovskites have a continuously tuneable band gap from 1 to 3 eV enabling applications far beyond photovoltaics.

Narrow thiophene-edged graphene nanoribbons (GNRs) were prepared from polychlorinated thiophene-containing poly(p-phenylene)s using the photochemical, metal-free cyclodehydrochlorination (CDHC) reaction. ¹H NMR and Raman spectroscopy confirmed the structures of the GNRs. The regioselectivity of the CDHC reaction allows the preparation of both laterally symmetrical and unsymmetrical GNRs and, consequently, the modulation of their optical and electronic properties.

GN'R light: Narrow thiophene-edged graphene nanoribbons (GNRs) were prepared from polychlorinated thiophene-containing poly(p-phenylene)s using photochemical, metal-free cyclodehydrochlorination (CDHC). The regioselectivity of the CDHC reaction allows the preparation of both longitudinally symmetrical and unsymmetrical GNRs and, consequently, modulation of their optical and electronic properties.

Ultrathin nanostructures are attractive for diverse applications owing to their unique properties compared to their bulk materials. Transition-metal chalcogenides are promising electrocatalysts, yet it remains difficult to make ultrathin structures (sub-2 nm), and the realization of their chemical doping is even more challenging. Herein we describe a soft-template mediated colloidal synthesis of Fe-doped NiSe₂ ultrathin nanowires (UNWs) with diameter down to 1.7 nm. The synergistic interplay between oleylamine and 1-dodecanethiol is crucial to yield these UNWs. The in situ formed amorphous hydroxide layers that is confined to the surface of the ultrathin scaffolds enable efficient oxygen evolution electrocatalysis. The UNWs exhibit a very low overpotential of 268 mV at 10 mA cm⁻² in 0.1 m KOH, as well as remarkable long-term stability, representing one of the most efficient noble-metal-free catalysts.

Down to the wire: Colloidal Fe-doped NiSe₂ ultrathin nanowires (UNWs) down to 1.7 nm in diameter were synthesized by a binary soft-template strategy. These UNWs yield surface-confined electrochemical oxidation, enabling efficient and robust oxygen evolution catalysis owing to their favorable electronic structures and unsaturated local coordination environments.

A rigid, inherently chiral bilayer nanographene has been synthesized as both the racemate and enantioenriched M isomer (with 93 % ee) in three steps from established helicenes. This folded nanographene is composed of two hexa-peri-hexabenzocoronene layers fused to a [10]helicene, with an interlayer distance of 3.6 Å as determined by X-ray crystallography. The rigidity of the helicene linker forces the layers to adopt a nearly aligned AA-stacked conformation, rarely observed in few-layer graphene. By combining the advantages of nanographenes and helicenes, we have constructed a bilayer system of 30 fused benzene rings that is also chiral, rigid, and remains soluble in common organic solvents. We present this as a molecular model system of bilayer graphene, with properties of interest in a variety of potential applications.

A twist of graphene: An inherently chiral bilayer nanographene with a helicene linker has been synthesized as the racemate and the M isomer with 93 % ee. This first member of a new family of twisted double-layer polyaromatic hydrocarbons is characterized, and its electrochemical and photophysical properties explored.

Major disadvantages of black phosphorus (BP) are its poor air-stability and poor solubility in common organic solvents. The best way to solve this problem is to incorporate BP into a polymer backbone or a polymer matrix to form novel functional materials that can provide both challenges and opportunities for new innovation in e.g., optoelectronic and photonic applications. As a proof-of concept application, we synthesized in situ the first highly soluble conjugated polymer-covalently functionalized BP derivative (PDDF-g-BP) which was used to fabricate a resistive random access memory (RRAM) device with a configuration of Au/PDDF-g-BP/ITO. In contrast to PDDF without memory effect, PDDF-g-BP-based device exhibits a nonvolatile rewritable memory performance, with a turn-on and turn-off voltages of +1.95V and -2.34V, and an ON/OFF current ratio of 10000. The current through the device in both the ON and OFF states is still kept unchanged even at 200th switching cycle. The PDDF/BP blends show a very unstable memory performance with a very small ON/OFF current ratio.

Extraordinary electronic and photonic features render black phosphorus (BP) an important material for the development of novel electronics and optoelectronics. Despite recent progress in the preparation of thinly layered BP flakes, scalable synthesis of large-size, pristine BP flakes remains a major challenge. An electrochemical delamination strategy is demonstrated that involves intercalation of diverse cations in non-aqueous electrolytes, thereby peeling off bulk BP crystals into defect-free flakes comprising only a few layers. The interplay between tetra-n-butylammonium cations and bisulfate anions promotes a high exfoliation yield up to 78 % and large BP flakes up to 20.6 μm. Bottom-gate and bottom-contact field-effect transistors, comprising single BP flakes only a few layers thick, exhibit a high hole mobility of 252±18 cm² V⁻¹ s⁻¹ and a remarkable on/off ratio of (1.2±0.15)×10⁵ at 143 K under vacuum. This efficient and scalable delamination method holds great promise for development of BP-based composites and optoelectronic devices.

Defect-free black phosphorus (BP) flakes were prepared on a macroscopic scale by a simple and scalable exfoliation method. Delaminated and mechanically exfoliated BP flakes present comparable electronic properties. This efficient and scalable method holds promise for development of BP-based composites and optoelectronic devices.

Colloidal semiconductor nanocrystals (SCNCs) or, more broadly, colloidal quantum nanostructures constitute outstanding model systems for investigating size and dimensionality effects. Their nanoscale dimensions lead to quantum confinement effects that enable tuning of their optical and electronic properties. Thus, emission color control with narrow photoluminescence spectra, wide absorbance spectra, and outstanding photostability, combined with their chemical processability through control of their surface chemistry leads to the emergence of SCNCs as outstanding materials for present and next-generation displays. In this Review, we present the fundamental chemical and physical properties of SCNCs, followed by a description of the advantages of different colloidal quantum nanostructures for display applications. The open challenges with respect to their optical activity are addressed. Both photoluminescent and electroluminescent display scenarios utilizing SCNCs are described.

Colloidal quantum nanostructures constitute outstanding model systems for investigating size and dimensionality effects. Their nanoscale dimensions lead to quantum confinement effects that enable tuning of their optical and electronic properties. This Review presents current and potential applications of semiconductor nanocrystals as sophisticated materials for display technologies.

Two-dimensional boron sheets (borophenes) have been successfully synthesized in experiments and are expected to exhibit intriguing transport properties. A comprehensive first-principles study is reported of the intrinsic electrical resistivity of emerging borophene structures. The resistivity is highly dependent on different polymorphs and electron densities of borophene. Interestingly, a universal behavior of the intrinsic resistivity is well-described using the Bloch–Grüneisen model. In contrast to graphene and conventional metals, the intrinsic resistivity of borophenes can be easily tuned by adjusting carrier densities, while the Bloch–Grüneisen temperature is nearly fixed at 100 K. This work suggests that monolayer boron can serve as intriguing platform for realizing tunable two-dimensional electronic devices.

The intrinsic resistivity of borophene is highly dependent on the polymorphs and the carrier densities. The resistivity is well-described using the Bloch–Grüneisen model, and it exhibits a universal scaling behavior.

Crystalline solids with intrinsically low lattice thermal conductivity (κ_L) are crucial to realizing high-performance thermoelectric (TE) materials. Herein, we show an ultralow κ_L of 0.35 Wm⁻¹ K⁻¹ in AgCuTe, which has a remarkable TE figure-of-merit, zT of 1.6 at 670 K when alloyed with 10 mol % Se. First-principles DFT calculation reveals several soft phonon modes in its room-temperature hexagonal phase, which are also evident from low-temperature heat-capacity measurement. These phonon modes, dominated by Ag vibrations, soften further with temperature giving a dynamic cation disorder and driving the superionic transition. Intrinsic factors cause an ultralow κ_L in the room-temperature hexagonal phase, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high-temperature rocksalt phase. Despite the cation disorder at elevated temperatures, the crystalline conduits of the rigid anion sublattice give a high power factor.

Low thermal conduction: Soft phonon modes and optical-acoustic phonon coupling cause an ultralow lattice thermal conductivity in the room-temperature hexagonal phase of AgCuTe, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high-temperature rocksalt phase. A high thermoelectric figure of merit (zT) of 1.6 is achieved in the p-type AgCuTe at around 670 K.

Self-assembly of nanoparticles provides unique opportunities as nanoplatforms for controlled delivery. By exploiting the important role of non-covalent hydrophobic interactions in the engineering of stable assemblies, nanoclusters were formed by self-assembly of fluorinated quantum dots in aqueous medium through fluorine-fluorine interactions. These nanoclusters were able to encapsulate different enzymes (laccase and α-galactosidase) obtaining encapsulation efficiencies ≥ 74 %. Importantly, the encapsulated enzymes maintained their catalytic activity, following a Michaelis-Menten kinetics. Under acidic environment the nanoclusters were slowly disassembled allowing the release of encapsulated enzymes. The effective release of the assayed enzymes demonstrated the feasibility of this nanoplatform to be used in pH-mediated enzyme delivery. In addition, the as-synthesized nanoclusters of ca. 50 nm diameter presented high colloidal stability and fluorescence emission, which make them a promising multifunctional nanoplatform.

Hydrogen bonding has been employed as a powerful tool for developing multi-component assemblies such as supramolecular polymers. We have prepared graphene quantum dot (GQD)-organic hybrid compounds (GQD-2b-e) by introducing 3,4,5-tri(hexadecyloxy)benzyl groups (C16) and linear chains terminated with a 2-ureido-4-[1H]-pyrimidione (UPy) moiety onto the periphery of GQD-1. GQD-2b-e formed supramolecular assemblies through hydrogen bonding between the UPy units. GPC analysis showed that GQDs with high loadings of the UPy group formed larger assemblies, and this trend was confirmed by DOSY and viscosity measurements. AFM images showed the polymeric network structures of GQD-2e on mica with flat structures (approximately 1.1 nm in height), but no such structures were observed in GQD-2a, which only carries the C16 group. We found that GQD-2c and GQD-2d formed organogels in n-decanol, and the gelation properties can be altered by replacing the alkyl chains in the UPy group with ethylene glycol chains (GQD-3). These results demonstrate that GQD can be used as a platform for supramolecular polymers and organogelators by suitable chemical functionalization.

Lead-free perovskite nanocrystals (NCs) were obtained mainly by substituting a Pb2+ cation with a divalent cation or substituting three Pb2+cations with two trivalent cations. Herein we report the substitution of two Pb2+cations with one monovalent Ag+ and one trivalent Bi3+ cations to synthesize Cs2AgBiX6 (X= Cl, Br, I) double perovskite NCs. Using femtosecond transient absorption spectroscopy, we have elucidated the charge carrier relaxation mechanism in the double perovskite NCs. The Cs2AgBiBr6 NCs exhibit ultrafast hot-carrier cooling (<1 ps), which competes with the carrier trapping processes (mainly originate from the surface defects). Notably, the photoluminescence can be increased by 100 times with surfactant (oleic acid) added to passivate the defects in Cs2AgBiCl6 NCs. These results suggest that the double perovskite NCs could be potential materials for optoelectronic applications by better controlling the surface defects.