F4TCNQ Molecules Research Articles

Gate-Switchable Molecular Diffusion on a Graphene Field-Effect Transistor.

Controlling the surface diffusion of particles on 2D devices creates opportunities for advancing microscopic processes such as nanoassembly, thin-film growth, and catalysis. Here, we demonstrate the ability to control the diffusion of F4TCNQ molecules at the surface of clean graphene field-effect transistors (FETs) via electrostatic gating. Tuning the back-gate voltage (VG) of a graphene FET switches molecular adsorbates between negative and neutral charge states, leading to dramatic changes in their diffusion properties. Scanning tunneling microscopy measurements reveal that the diffusivity of neutral molecules decreases rapidly with a decreasing VG and involves rotational diffusion processes. The molecular diffusivity of negatively charged molecules, on the other hand, remains nearly constant over a wide range of applied VG values and is dominated by purely translational processes. First-principles density functional theory calculations confirm that the energy landscapes experienced by neutral vs charged molecules lead to diffusion behavior consistent with experiment. Gate-tunability of the diffusion barrier for F4TCNQ molecules on graphene enables graphene FETs to act as diffusion switches.

Simple ultrasonic mixing to obtain charge transfer dopants of graphene oxide for highly responsive photodetectors and efficient image extraction

Regulating the charge-related properties in materials is highly valuable in the field of electronic devices. Charge transfer doping received significant attention as an efficient method of modulation. Graphene oxide (GO) is often used for regulation due to its abundance of oxygen-containing groups. However, the conventional operations to perform charge transfer doping for GO are complex, expensive and environmentally unfriendly. Here, we propose a charge transfer doping strategy by simply mixing GO suspension with 5 mg/ml 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ), together with ultrasonic treatment. Notably, the F4TCNQ-GO flexible photodetector exhibits outstanding responsivity (R) of up to 1.57 × 103 A/W in the 650 nm regime, surpassing most of the GO/graphene-based photodetectors reported in the past decade. Moreover, the photodetector successfully realizes in-sensor computing for complex pattern imaging, enabling clear and efficient multi-directional edge extraction. Experiments employing X-ray photoelectron spectra (XPS), X-ray diffraction spectra (XRD), Raman spectra, Ultraviolet–visible (UV–vis) spectra and scanning electron microscopy (SEM) reveal the stable non-covalent binding between GO and F4TCNQ through π-π interactions, as well as the electron transfer from GO to the F4TCNQ molecule, resulting in the formation of electron accumulation and depletion layers on the F4TCNQ molecule and GO, respectively. This finding holds great potential for regulating the electronic properties of a wide range of π-electron-rich two-dimensional materials in the fields of energy, biomedicine, environmental technology, and materials science.

Structure Determination of F4TCNQ on Ag(111): A Systematic Trend in Metal Adatom Incorporation.

A structure determination of the commensurate phase formed by 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (F4TCNQ) absorbed on Ag(111) is reported. Initial characterization was performed using low-energy electron diffraction and synchrotron radiation photoelectron spectroscopy, with quantitative structural data being provided by normal incident X-ray standing waves (NIXSW) and surface X-ray diffraction (SXRD). NIXSW data show the F4TCNQ molecule to adopt a "twisted" conformation on the surface, previously found to be associated with metal adatom incorporation into a 2d-metal-organic framework for F4TCNQ on Au(111), Ag(100), and Cu(111). SXRD results provide direct evidence of the presence of Ag adatoms that are found to occupy near-bridge or fcc hollow sites with respect to the underlying surface, at an adsorption height of 2.69 ± 0.10 Å. The results show a consistent pattern of behavior for F4TCNQ adsorption on the (111) surfaces of Cu, Ag, and Au.

Two-dimensional monolayer from organic molecules F4-TCNQ via DFT calculations

Conjugated microporous polymers is a class of organic nanoporous polymers in which the pores can be easily controlled by molecule design. Here we present a new class of 2D structure based on F4-TCNQ molecules. Using ab initio calculations, the possibilities for the formation of such 2D materials based on F4-TCNQ molecules are studied as well. Among considered structures only two were selected as the most preferable for 2D layer formation. Predicted monolayers were analyzed on thermal stability, electronic, optical and mechanical properties. Combination of stability, elasticity and semiconducting band gap make suggested monolayers promising candidate for fabrication of elements of flexible and controllable optoelectronics.

Study of Formation and Properties of F4TNCQ/MoS2 Heterostructures with point defects driven by DFT and Neural Network Potential

State-of-the-art computational techniques allow us to perform the comprehensive investigation of organic/inorganic heterostructures based on F4TCNQ molecule and MoS2 monolayer. The features of electronic and optical properties with the presence of point defects at MoS2 surface were studied. We found that halogen impurities lead to strengthening of organic/inorganic interface. Based on variation of the binding energy between F4TCNQ and MoS2 surface the neural network potential was trained with the aim to study the possibility of formation of monomolecular layer. Molecular dynamics calculation shows the anchoring behavior of F4TCNQ molecule in a position of sulfur vacancy with the same binding energy as in case of halogen doping. Present results show advantages of defect creation in MoS2 surface with the aim to control the ordering of monomolecular layer and the properties of the whole heterostructure. We show the first fundamental step of using organic/inorganic heterostructures for application in optoelectronic sensors and detectors.

Molecular p-doping induced dielectric constant increase of polythiophene films determined by impedance spectroscopy

The dielectric constant (εr) is a fundamental material parameter that governs charge transfer processes in organic semiconductors, yet its value is often assumed rather than measured. Here, we use impedance spectroscopy to determine εr in regioregular poly(3-hexylthiophen-2,5-diyl) (P3HT) thin films p-doped with the molecular dopants hexafluoro-tetracyanonaphthoquinodimethane and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). We fit the impedance spectra using a single RC circuit model to determine the frequency-dependent capacitance and extract εr. The value of the dielectric constant increases by around two-thirds from 2.9 ± 0.1 (undoped polymer) to 4.9 ± 0.6 on the addition of one F4TCNQ molecule per 500 P3HT monomer units. In contrast, the addition of the weak dopant 7,7,8,8-tetracyanoquinodimethane (TCNQ), which does not undergo ground state charge transfer with P3HT, has no effect on the dielectric constant. Our results support the hypothesis that molecular doping has a considerable impact on the materials dielectric constant via polarizable host-dopant complexes.

F4-TCNQ on Epitaxial Bi-Layer Graphene: Concentration- and Orientation-Dependent Charge Transfer at the Interface

Bi-layer epitaxial graphene (BLG) on 6H-SiC(0001) (EG/SiC) was grown and modified by thermal deposition of the molecular electron acceptor tetrafluoro-tetra cyano quinodimethane (F4-TCNQ). The surface-modified system, F4-TCNQ/EG/SiC, was studied by X-ray photoelectron spectroscopy (XPS) and angle-resolved polarized Raman spectroscopy (ARPRS). XPS results indicate that bonding of deposited F4-TCNQ molecules depends on their concentration. Although bonding through the cyano groups is present at all concentrations, charge transfer from graphene to fluorine is evident only at sub-monolayer concentrations. The corresponding change in bond character is coupled with a change in molecular orientation. Raman spectroscopy not only provides results consistent with the findings from the XPS study but also reveals a significant degree of molecular stacking above the monolayer concentration. Thus, both the variation of the acceptor concentration and the number of graphene layers provide further handles to manipulate charge and doping that may be useful in device applications.

Femtosecond nonlinear optical response and minority carrier lifetime of F4TCNQ-doped bismuthene for optoelectronic and ultrafast photonic applications

Bismuthene has received a lot of interest because of its unusual electronic and photonic properties. To widen the use of bismuthene in optoelectronic and ultrafast photonic devices, its electronic and optical characteristics should be tuned. Herein, for the first time, a strong p-type molecular dopant, such as tetrafluoro-tetracyanoquinodimethane (F4TCNQ), is used to achieve efficient surface charge transfer doping (SCTD) of bismuthene. The minority carrier lifetime measurements of bismuthene shifted to 6.5 µs after doping, confirming that F4TCNQ has the opportunity to boost bismuthene's p-type conductivity due to an unusual charge transfer interaction between bismuthene and F4TCNQ molecules. The thermally generated intermolecular charge transfer influence between bismuthene and F4TCNQ resulted in an improved nonlinear optical and optical limiting response, which was estimated by femtosecond z-scan measurements. Our findings suggest that SCTD is an appropriate means for tuning the linear and nonlinear optical properties of bismuthene via modulating their band gap and minority carrier lifetime. This alternating approach expands its potential in electronic, optoelectronic, and ultrafast photonic devices.

Enhanced thermoelectric performance of holey silicon thin films using F4TCNQ surface doping

Silicon thin films have great potential as chip-integrated Peltier micro-coolers and thermoelectric power generators due to their industry compatibility and cost effectiveness. Improving the thermoelectric figure of merit, zT, and therefore the device efficiency can be achieved by increasing the power factor while decreasing the thermal conductivity. In this work, we study single crystalline silicon thin films with patterned nano-holes with sizes comparable to the phonon mean free path to suppress thermal conductivity. The holey silicon thin films are then surface doped with organic molecules F4TCNQ to create a three-dimensional modulation doping scheme. As the dopants are outside the host material, there is less impurity scattering, which improves carrier mobility and the overall power factor. We fabricate silicon thin films with periodic arrays of nano-sized holes, with a fixed pitch size of 300 nm. By changing the hole diameters, we vary the neck size from 169 nm to 22 nm. The in-plane thermal conductivity, measured using the heat diffusion imaging method, demonstrates an order of magnitude reduction compared to bulk silicon and a change from 26 Wm−1K−1 to 5 Wm−1K−1 at room temperature. The films with large hole diameters allow space for the relatively large F4TCNQ molecules and hence effective surface doping, which is evident by orders of magnitude improvement in the electrical conductivity and power factor.

Novel fabrication process of charge transfer complex thin films for large area by thermal diffusion and their n-channel organic transistor operation

We have proposed a new fabrication process of donor and acceptor charge transfer complex thin films using thermal diffusion. 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ) was vacuum-deposited on 2,7-didecylbenzothienobenzothiophene (C10-BTBT) thin films fabricated by spin-coating, and F4-TCNQ molecules were diffused into C10-BTBT thin films by thermally annealing. The X-ray diffraction pattern and field effect transistor of thermally annealed thin films indicated the crystal structure of the charge transfer complex of C10-BTBT and F4-TCNQ and n-channel operation in air, respectively. We conclude that it is good process to use the thermal diffusion for the fabrication of the large area thin films of charge transfer complex.

Segregated Array Tailoring Charge-Transfer Degree of Organic Cocrystal for the Efficient Near-Infrared Emission beyond 760nm.

Harvesting the narrow bandgap excitons of charge-transfer (CT) complexes for the achievement of near-infrared (NIR) emission has attracted intensive attention for its fundamental importance and practical application. Herein, the triphenylene (TP)-2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4 TCNQ) CT organic complex is designed and fabricated via the supramolecular self-assembly process, which demonstrates the NIR emission with a maximum peak of 770nm and a photoluminescence quantum yield (PLQY) of 5.4%. The segregated stacking mode of TP-F4 TCNQ CT complex based on the multiple types of intermolecular interaction has a low CT degree of 0.00103 and a small counter pitch angle of 40° between F4 TCNQ and TP molecules, which breaks the forbidden electronic transitions of CT state, resulting in the effective NIR emission. Acting as the promising candidates for the active optical waveguide in the NIR region beyond 760nm, the self-assembled TP-F4 TCNQ single-crystalline organic microwires display an ultralow optical-loss coefficient of 0.060dBµm-1 . This work holds considerable insights for the exploration of novel NIR-emissive organic materials via an universal "cocrystal engineering" strategy.

Structural insights into Lewis acid- and F4TCNQ-doped conjugated polymers by solid-state magnetic resonance spectroscopy.

Molecular doping strategies facilitate orders of magnitude enhancement in the charge carrier mobility of organic semiconductors (OSCs). Understanding the different doping mechanisms and molecular-level constraints on doping efficiency related to the material energy levels is crucial to develop versatile dopants for OSCs. Given the compositional and structural heterogeneities associated with OSC thin films, insight into dopant-polymer interactions by long-range techniques such as X-ray scattering and electron microscopy is exceedingly challenging to obtain. This study employs short-range probes, solid-state (ss)NMR and EPR spectroscopy, to resolve local structures and intermolecular interactions between dopants such as F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), Lewis acid BCF (tris[pentafluorophenyl] borane) and Lewis base conjugated polymer, PCPDTBT (P4) (poly[2,6-(4,4-bis(2-hexadecyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]). Analysis of 1H and 13C ssNMR spectra of P4, P4 : F4TCNQ and P4 : BCF blends indicates that the addition of dopants induces local structural changes in the P4 polymer, and causes paramagnetism-induced signal broadening and intensity losses. The hyperfine interactions in P4 : BCF and P4 : F4TCNQ are characterized by two-dimensional pulsed EPR spectroscopy. For P4 : F4TCNQ, 19F ssNMR analysis indicates that the F4TCNQ molecules are distributed and aggregated into different local chemical environments. By comparison, BCF molecules are intermixed with the P4 polymer and interact with traces of water molecules to form BCF-water complexes that serve as Brønsted acid sites, as revealed by 11B ssNMR spectroscopy. These results indicate that the P4-dopant blends exhibit complex morphology with different distributions of dopants, whereby the combined use of ssNMR and EPR provides essential insights into how higher doping efficiency is observed with BCF and a mediocre efficiency is associated with F4TCNQ molecules.

Position‐Induced Efficient Doping for Highly Doped Organic Thermoelectric Materials

AbstractElectrical doping is essentially required for high‐performance organic thermoelectric (TE) materials; however, the doping efficiency ηd has not been extensively investigated in highly doped organic semiconductors (OSCs). Here, it is demonstrated that the distribution of dopant molecules in a specific position in highly doped OSCs affects the ηd, which is critically related to the Seebeck coefficient S and the electrical conductivity σ. Poly(2,5‐bis(3‐hexadecylthiophen‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT) films are p‐doped with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) by either solution‐sequential (SSq) doping or vapor doping. SSq doping deposited F4TCNQ only in the amorphous domains of PBTTT films, whereas vapor doping deposited it in both the amorphous and crystalline domains. F4TCNQ molecules in the crystalline domains exhibited a high ηd and led to a rapid increase of the power factor with increasing σ: S2σ ∝ σ0.76. These results provide guidance for the efficient doping of highly doped OSCs and emphasize the importance of doping efficiency in obtaining high‐performance organic TE materials.

Microsecond dynamics of molecular negative ions formed by low-energy electron attachment to fluorinated tetracyanoquinodimethane.

Low-energy (0-15eV) electron interactions with gas-phase 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) molecules are studied under single collision conditions using dissociative electron attachment spectroscopy. The experimental findings are supported by density functional theory calculations of the virtual orbital energies and energetics of the dissociative decays. Long-lived molecular negative ions F4-TCNQ- are detected in a wide electron energy range (0-3eV) with electron detachment times in the range of milliseconds. Although plenty of decay channels are observed, their intensities are found to be very small (two to four orders of magnitude relative to the F4-TCNQ- signal). These findings prove that the structure of this strong electron-accepting molecule bearing an excess electron is robust in its electronic ground state, even when highly (up to 6eV) vibrationally excited. As many as nine metastable fragment anions formed slowly (in the 16-23 µs range) are found in the negative ion mass spectrum of F4-TCNQ, as never observed before in compounds possessing high electron-accepting ability. The present results shed some light on microsecond dynamics of isolated F4-TCNQ molecules under conditions of excess negative charge, which are important for understanding the functionality of nanoscale devices containing this molecule as a structural element.

Imaging Reconfigurable Molecular Concentration on a Graphene Field-Effect Transistor.

The spatial arrangement of adsorbates deposited onto a clean surface under vacuum typically cannot be reversibly tuned. Here we use scanning tunneling microscopy to demonstrate that molecules deposited onto graphene field-effect transistors (FETs) exhibit reversible, electrically tunable surface concentration. Continuous gate-tunable control over the surface concentration of charged F4TCNQ molecules was achieved on a graphene FET at T = 4.5K. This capability enables the precisely controlled impurity doping of graphene devices and also provides a new method for determining molecular energy level alignment based on the gate-dependence of molecular concentration. Gate-tunable molecular concentration is explained by a dynamical molecular rearrangement process that reduces total electronic energy by maintaining Fermi level pinning in the device substrate. The molecular surface concentration is fully determined by the device back-gate voltage, its geometric capacitance, and the energy difference between the graphene Dirac point and the molecular LUMO level.

Large difference of doping behavior of tetracyanoquinodimethane (TCNQ) and 2,3,5,6-Tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ) on field effect transistor with channel of atomic layer MoS2

We examined the mechanism of the large difference of the doping effect of two molecules of tetracyanoquinodimethane(TCNQ) and 2,3,5,6-Tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ) on the field effect transistor (FET) property, which is composed of the atomically thin MoS2 channel. The electron-acceptor behaviors of the two molecules are detected by examining the threshold voltage shift (ΔVth) of the drain current vs gate voltage curve plot. It shows only a mild shift of ∼4 V for the TCNQ but it shows a large value of ∼33 V for the F4-TCNQ molecule. The similar rapid change with small amount of F4-TCNQ deposition was detected as the Fermi level shift in the ultra violet photoemission spectroscopy (UPS). The DFT calculation shows the flat lying configuration for both molecules after the structural optimization, and the difference of the charge transfer to the substrate is not large enough to explain the difference of theΔVth. Instead, we want to propose a model in which the bonding of the F atom of the F4-TCNQ molecule at the S-defect contribute to the initial rapid variation of theΔVth.

Modulating the Electronic, Optical, and Transport Properties of CdTe and ZnTe Nanostructures with Organic Molecules: A Theoretical Investigation.

In this paper, we systematically investigated the electronic, optical, and transport properties of CdTe and ZnTe nanostructures before and after adsorption with benzyl viologen (BV) and tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) organic molecules based on the first principles calculation. First, the band gaps of CdTe and ZnTe nanostructures obviously decrease after BV and F4-TCNQ adsorptions. Interestingly, the electronic property calculation shows that BV and F4-TCNQ can donate/accept electrons to/from the surface of CdTe and ZnTe nanostructures, leading to an effective n-/p-type doping, respectively. Second, the optical absorption in a broad spectral range (from visible to near-infrared) of CdTe and ZnTe is significantly improved by adsorption of BV and F4-TCNQ molecules, offering great opportunities for the use of CdTe and ZnTe nanostructures in renewable energy fields. Lastly, the electrical transfer characteristics on CdTe and ZnTe nanostructure-based field-effect transistors clearly showed that the conduction of the nanostructures can be rationally tuned into n- and p-type conductivity with BV and F4-TCNQ adsorptions, respectively. Our work clearly demonstrates that the electronic, optical, and transport properties of CdTe and ZnTe nanostructures are effectively modulated by adsorption of BV and F4-TCNQ, which can be used to construct high-performance electronic and optoelectronic devices.

Effective passivation of black phosphorus against atmosphere by quasi-monolayer of F4TCNQ molecules

Black phosphorus (BP) has drawn extensive attention due to its unique semiconducting properties, but the poor stability of BP greatly limits its practical device application. In this work, we have fabricated a passivation layer of quasi-monolayer 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) on the BP simply by vacuum evaporation and annealing. The desirable air stability and strong interface charge transfer (ICT) of quasi-monolayer F4TCNQ/BP were confirmed with photoemission spectroscopy (PES) characterization. Density functional theory (DFT) calculations were also applied to further investigate the passivation mechanism, and the results show that the lone pair electrons in BP's valence band transfer to F4TCNQ molecules and were firmly localized due to the strong withdrawing ability of F4TCNQ, which greatly enhanced the energy barrier of electrons transfer to H2O and O2 and hindered the further oxidation of phosphorus atoms. Meanwhile, a nearly Ohmic contact is formed across the F4TCNQ/BP interface which may greatly facilitate the carrier transport in BP based devices.

Gas adsorption effects on electronic and magnetic properties of triangular graphene antidot lattices

The adsorption effects of small molecules (H2O, CO, NH3, NO2) and large molecules (Tetracyanoquinodimethane (TCNQ) and Tetrafluoro-tetracyanoquinodimethane (F4TCNQ)) on electronic and magnetic properties of two triangular graphene antidot lattices (GALs), [10, 3, 6]RTA and [10, 5]ETA, are investigated by means of first-principles calculations. We find that CO, NO2, TCNQ, and F4TCNQ molecules are chemisorbed by both antidots, whereas NH3 is physisorbed (chemisorbed) by [10, 5]ETA ([10, 3, 6]RTA) structure. H2O, CO, NH3 molecules reveal no significant effect on electronic and magnetic properties of these antidot structures. The adsorbed NO2 molecules affect the energy gap of GALs by changing their electronic structure from semiconducting to half-metal nature. This suggests that both GALs can act as efficient NO2 sensors. The adsorption of TCNQ and F4TCNQ molecules on GALs induces flat bands in the vicinity of the Fermi energy and also turn the electronic structure of antidot lattices to half-metallicity. Among the small and large molecules, NO2 molecules induce the most total magnetic moment, paving the way to make magnetic GAL-based devices.

Molecular doping of blue phosphorene: a first-principles investigation

Using first-principles calculations, we show that p-doped blue phosphorene can be obtained by molecular doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F6-TNAP), whereas n-doped blue phosphorene can be realized by doping with tetrathiafulvalene (TTF) and cyclooctadecanonaene (CCO). Moreover, the doping gap can be effectively modulated in each case by applying an external perpendicular electric field. The optical absorption of blue phosphorene can be considerably enhanced in a broad spectral range through the adsorption of CCO, F4-TCNQ, and F6-TNAP molecules, suggesting potential of the doped materials in the field of renewable energy.