Two-dimensional Molecular Crystals Research Articles

Molecularly Thin 2D Organic Single Crystals: A New Platform for High-Performance Polarization-Sensitive Phototransistors.

The inherent linear dichroism (LD), high absorption, and solution processability of organic semiconductors hold immense potential to revolutionize polarized light detection. However, the disordered molecular packing inherent to polycrystalline thin films obscures their intrinsic diattenuation, resulting in diminished polarization sensitivity. In this study, we develop filter-free organic polarization-sensitive phototransistors (PSPs) with both a high linear dichroic ratio (LDR) and exceptional photosensitivity utilizing molecularly thin dithieno[3,2-b:2',3'-d]thiophene derivatives (DTT-8) two-dimensional molecular crystals (2DMCs) as the active layer. The orderly molecular packing in 2DMCs amplifies the inherent LD, and their molecular-scale thickness enables complete channel depletion, significantly reducing the dark current. As a result, PSPs with an impressive LDR of 3.15 and a photosensitivity reaching 3.02 × 106 are obtained. These findings present a practical demonstration of using the polarization angle as an encryption key in optical communication, showcasing the potential of 2DMCs as a viable and promising category of semiconductors for filter-free, polarization-sensitive photodetectors.

Emergence of Two-Dimensional Inorganic Molecular Crystals

Emergence of Two-Dimensional Inorganic Molecular Crystals

Organic field-effect transistors based on two-dimensional molecular crystals of thiophene/selenophene-based triacene asymmetric derivatives as active layer

From 2,5-di(trimethylsilanyl)dithieno[2,3-b:3′,2′-d]thiophene ((TMS)2-DTT) and (TMS)2-diseleno[2,3-b:3′,2′-d]thiophene ((TMS)2-DST), asymmetric organic semiconductor materials (E)− 2-(4-octylstyryl)− 5-phenyl-DTT (PEP-DTT) and (E)− 2-(4-octylstyryl)− 5-phenyl-DST (PEP-DST) were synthesized via bromination, Suzuki coupling, deprotection, formylation and Wittig reactions, with the yields of 48% and 28%, respectively. Apart from the synthetic work, two-dimensional molecular crystals (2DMCs) with a growth size of few hundreds of micrometers of PEP-DTT and PEP-DST were successfully realized for organic field-effect transistors (OFETs) devices through “layer-defining strategy”. Bottom-gate/top-contact OFETs on the basis of PEP-DTT and PEP-DST 2DMCs as active layers displayed p-channel features with maximum mobility of 2.04 and 1.36 cm2 V−1 s−1, respectively.

Structural diversity and topological property of I-based two-dimensional inorganic molecular crystals

Structural diversity and topological property of I-based two-dimensional inorganic molecular crystals

Large-scale surface hopping simulation of charge transport in hexagonal molecular crystals: role of electronic coupling signs

In large-scale surface hopping simulations with a huge number of electronic states, trivial crossings could easily lead to incorrect long-range charge transfer and induce large numerical errors. We here study the charge transport in two-dimensional hexagonal molecular crystals with a parameter-free full crossing corrected global flux surface hopping method. Fast time-step size convergence and system size independence have been realized in large systems containing thousands of molecular sites. In hexagonal systems, each molecular site has six nearest neighbours. We find that the signs of their electronic couplings have a strong impact on the charge mobility and delocalization strength. In particular, changing the signs of electronic couplings can even lead to a transition from hopping to band-like transport. In comparison, such phenomena cannot be observed in extensively studied two-dimensional square systems. This is attributed to symmetry of the electronic Hamiltonian and distribution of the energy levels. Due to its high performance, the proposed approach is promising to be applied to more realistic and complex systems for molecular design.

Molecular layer modulation of two-dimensional organic ferroelectric transistors

Ferroelectric transistors hold great potential in low consumption devices. Due to the high film quality and clean system, two dimensional organic semiconductors are widely employed to fabricate high performance organic electronic devices and explore the modulation mechanism of the molecular packing on device performance. Here, we combine the ferroelectric hafnium oxide HfZrO x and two-dimensional molecular crystal 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2b]thiophene (C10-DNTT) with controllable layers to study the molecular layer modulation of ferroelectric organic thin-film transistors (OTFTs). The contact resistance, driving current and transconductance are directly affected by the additional access resistance across the upper molecular layers at the source/drain contact region. Simultaneously, the capacitance of Schottky junction related to the molecular layer thickness could effectively adjust the gate potential acting on the organic channel, further controlling the devices’ subthreshold swing and transconductance efficiency. This work would promote the development of low voltage and high performance OTFTs.

Giant Tunability of Charge Transport in 2D Inorganic Molecular Crystals by Pressure Engineering.

The unique intermolecular van der Waals force in emerging two-dimensional inorganic molecular crystals (2DIMCs) endows them with highly tunable structures and properties upon applying external stimuli. Using high pressure to modulate the intermolecular bonding, here we reveal the highly tunable charge transport behavior in 2DIMCs for the first time, from an insulator to a semiconductor. As pressure increases, 2D α-Sb2 O3 molecular crystal undergoes three isostructural transitions, and the intermolecular bonding enhances gradually, which results in a considerably decreased band gap by 25 % and a greatly enhanced charge transport. Impressively, the in situ resistivity measurement of the α-Sb2 O3 flake shows a sharp drop by 5 orders of magnitude in 0-3.2 GPa. This work sheds new light on the manipulation of charge transport in 2DIMCs and is of great significance for promoting the fundamental understanding and potential applications of 2DIMCs in advanced modern technologies.

表面自由能对二维结晶的影响

Two-dimensional (2D) molecular crystals are emerging materials with fascinating optoelectronic properties. However, growing 2D organic single crystals is challenging because little is known about the factors that determine 2D crystallization. Herein, we investigated the relationship between 2D crystallization and the surface free energy of organic semiconductors. We proposed a term called surface free energy anisotropy factor (g) to measure the tendency of 2D crystallization. Taking two [1]benzothieno[3,2-b][1]benzothiophene derivatives as examples, we identified that the one with a larger g factor showed a higher tendency for 2D crystallization, irrespective of the external factors. This work provides a guideline for the rational design of organic semiconductors toward 2D crystallization.

Wall-Confined Spreading Dynamics on the Surface of Surfactant Solution.

A liquid spreading over another is a universal physical process in the nature, which was investigated by the scaling law to reveal the underlying mechanical mechanism over the decades. However, scaling laws are restricted to piecewise physical stages, respectively. It is a challenge to present a full physical picture for a dynamic spreading process covering a wide-spectrum speed. We propose a general wall-confined spreading dynamics (WCSD) model originating from molecular kinetic theory (MKT). It creatively illustrates the order and domination between driving energy and energy dissipation (or transfer) using a phase diagram according to theory and experiments. This work reveals the deep mechanical mechanism of WCSD which provides an indirect guidance on the solution processing methods of two-dimensional molecular crystals (2DMCs) growth.

Fabrication of Sb2O3 by an improved chemical reaction assisted vertical micro sublimation method and its saturable absorber performance

Two-dimensional molecular crystals (2DMCs) are emerging ideal materials for future high-performance optoelectronic devices. People are constantly exploring new methods to solve the problem of difficult growth. Here, we design an improved chemical reaction-assisted vertical micro sublimation method based on vertical micro sublimation for the growth of two-dimensional Sb2O3 inorganic molecular crystals for the first time. The saturation absorption characteristics of the self-made Sb2O3 2DMCs were systematically tested, and the Q-switched laser output characteristics of Sb2O3 2DMCs at 1 µm were verified by using Nd: GYAP mixed crystal. The maximum average output power of 231 mW was achieved at 1080.6 nm, corresponding shortest pulse with a duration of 472 ns and maximum pulse repetition rate of 376 kHz. The maximum single pulse energy and the maximum peak power of the laser output were 0.614 µJ and 1.3 W, respectively. The growth of other 2DMCs would be motivated and its potential applications in the field of ultrafast photonics would be expanded with our findings.

Emerging Two-Dimensional Inorganic Molecular Crystals: The Concept and Beyond.

The concept of two-dimensional (2D) inorganic molecular crystals (IMCs) was first introduced by Zhai and coauthors in 2019. In contrast to the layered structures of graphene-like 2D materials, 2D IMCs consist of tiny inorganic molecules bonded together through all-around van der Waals (vdW) interactions. Their structural peculiarities lead to some special behaviors and appealing properties in their synthesis and applications. In this Perspective, we first introduce the concept of 2D IMCs and present the very first synthesis of 2D IMCs using a surface-passivated growth approach. The special intermolecular effects between the inorganic molecules are also summarized. In addition, because of its molecular structure, a vdW film of IMCs can be facilely fabricated, which exhibits appealing potential in integrated 2D devices. More importantly, we give a general outlook for the further development of 2D IMCs with the goal of attracting more attention to this emerging research frontier.

Frustrated magnetic interactions in a cyclacene crystal

We study the emergence of magnetism and its interplay with structural properties in a two-dimensional molecular crystal of cyclacenes, using density functional theory (DFT). Isolated cyclacenes with an even number of fused benzenes host two unpaired electrons in two topological protected zero modes, at the top and bottom carbon rings that form the molecule. We show that, in the gas phase, electron repulsion promotes an open-shell singlet with strong intramolecular antiferromagnetic exchange. We consider a closed packing triangular lattice crystal phase and we find a strong dependence of the band structure and magnetic interactions on the rotation angle of the cyclacenes with respect to the crystal lattice vectors. The orientational ground state maximizes the intermolecular hybridization, yet local moments survive. Intermolecular exchange is computed to be antiferromagnetic, and DFT predicts a broken symmetry ${120}^{\ensuremath{\circ}}$ spin phase reflecting the frustration of the intermolecular spin coupling. Thus, the cyclacene crystal realizes a bilayer of two antiferromagnetically coupled $S=1/2$ triangular lattices. Our results provide a bottom-up route towards carbon based strongly correlated platforms in two dimensions.

Soft template-assisted self-assembly: a general strategy toward two-dimensional molecular crystals for high-performance organic field-effect transistors

A soft template-assisted self-assembly (STAS) strategy was developed to grow large-area high-quality 2D molecular crystals on a water surface irrespective of the surface tension of the solvents.

Extraordinary Photostability and Davydov Splitting in BN-Sandwiched Single-Layer Tetracene Molecular Crystals.

Two-dimensional molecular crystals have been beyond the reach of systematic investigation because of the lack or instability of their well-defined forms. Here, we demonstrate drastically enhanced photostability and Davydov splitting in single and few-layer tetracene (Tc) crystals sandwiched between inorganic 2D crystals of graphene or hexagonal BN. Molecular orientation and long-range order mapped with polarized wide-field photoluminescence imaging and optical second-harmonic generation revealed high crystallinity of the 2D Tc and its distinctive orientational registry with the 2D inorganic crystals, which were also verified with first-principles calculations. The reduced dielectric screening in 2D space was manifested by enlarged Davydov splitting and attenuated vibronic sidebands in the excitonic absorption and emission of monolayer Tc crystals. Photostable 2D molecular crystals and their size effects will lead to novel photophysical principles and photonic applications.

P-n heterojunctions composed of two-dimensional molecular crystals for high-performance ambipolar organic field-effect transistors

Bilayer p-n heterojunctions are promising structures to construct ambipolar organic field-effect transistors (aOFETs) for organic integrated circuits. However, due to the lack of effective strategies for high-quality p-n heterojunctions with clear interfaces, the performance of aOFETs is commonly and substantially lower than that of their unipolar counterparts, which hinders the development of aOFETs toward practical applications. Herein, a one-step solution crystallization strategy was proposed for the preparation of high-quality bilayer p-n heterojunctions. A mixed solution of a p- and an n-type organic semiconductor was dropped on a liquid substrate, and vertical phase separation occurred spontaneously during crystallization to produce bilayer p-n heterojunctions composed of molecularly thin two-dimensional molecular crystals. Due to the clear interface of the bilayer p-n heterojunctions, the maximum mobility (average mobility) reached 1.96 cm2 V−1 s−1 (1.12 cm2 V−1 s−1) for holes and 1.27 cm2 V−1 s−1 (0.61 cm2 V−1 s−1) for electrons in ambient air. So far as we know, these values were the highest among double-channel aOFETs measured in ambient air. This work provides a simple yet efficient strategy to construct high-quality bilayer p-n heterojunctions, which lays a foundation for their application in high-performance optoelectronic devices.

Few-layered two-dimensional molecular crystals for organic artificial visual memories with record-high photoresponse

Molecularly thin two-dimensional molecular crystals (2DMCs) were used to construct artificial visual memories with high photoresponse.

Ultra-thin two-dimensional molecular crystals grown on a liquid surface for high-performance phototransistors.

A novel strategy for the growth of molecularly thin two-dimensional molecular crystals (2DMCs) of organic semiconductors with poor solubility was developed. Large-area bilayer 2DMCs were grown on a liquid surface at elevated temperatures, with record mobility and superior photoresponse.

Two-dimensional molecular crystals: a review

<p indent="0mm">Two-dimensional molecular crystals (2DMCs) have characteristics of long-range order, free boundary and low defect density, which make them ideal candidates to build high-performance optoelectronic devices. The low-cost and large-area production of 2DMCs is a key to their applications. At present, many methods have been developed for the controlled preparation of 2DMCs in gas or liquid phases, and the unique optoelectronic properties of 2DMCs have been revealed. In this review, we summarized the preparation methods of 2DMCs and their applications in the devices such as field-effect transistors and phototransistors.

Two-dimensional inorganic molecular crystals

Two-dimensional molecular crystals, consisting of zero-dimensional molecules, are very appealing due to their novel physical properties. However, they are mostly limited to organic molecules. The synthesis of inorganic version of two-dimensional molecular crystals is still a challenge due to the difficulties in controlling the crystal phase and growth plane. Here, we design a passivator-assisted vapor deposition method for the growth of two-dimensional Sb2O3 inorganic molecular crystals as thin as monolayer. The passivator can prevent the heterophase nucleation and suppress the growth of low-energy planes, and enable the molecule-by-molecule lateral growth along high-energy planes. Using Raman spectroscopy and in situ transmission electron microscopy, we show that the insulating α-phase of Sb2O3 flakes can be transformed into semiconducting β-phase under heat and electron-beam irradiation. Our findings can be extended to the controlled growth of other two-dimensional inorganic molecular crystals and open up opportunities for potential molecular electronic devices.

Layer-Defining Strategy to Grow Two-Dimensional Molecular Crystals on a Liquid Surface down to the Monolayer Limit.

Two-dimensional molecular crystals (2DMCs) open a new door for the controllable growth of 2D materials by molecular design with a energy gap and solution processability. However, the growth of 2DMCs with defined molecular layers remains full of challenges. Herein, we report a novel method to produce various 2DMCs with a defined number of molecular layers. When the surface tension and viscosity are tuned to control the spreading of the solution on the liquid surface, large-area quasi-freestanding 2DMCs from bulk size down to the monolayer limit are obtained, which makes it possible to probe the intrinsic layer-dependent optoelectronic properties of organic semiconductors down to the physical limit, and paves the way for the application of 2DMCs in new optoelectronic devices and technologies.