Fluorine Coverage Research Articles

Modifying electronic and optical properties of violet phosphorus through variable fluorine coverage.

We present a comprehensive investigation of the electronic properties of fluorinated monolayer violet phosphorus using first-principles calculations. Our results reveal a strong dependence of the electronic properties on the different fluorine coverages of fluorination. As the fluorine coverage increases, monolayer violet phosphorus undergoes a significant transition from a wide direct bandgap semiconductor to a narrow indirect bandgap semiconductor. Moreover, both semi-fluorinated and fully fluorinated monolayer violet phosphorus exhibit advantageous semiconducting characteristics, with a tunable bandgap of 0.50 ~ 1.04 eV under biaxial strain ranging from -6% to 6%. Notably, the fully fluorinated monolayer violet phosphorus demonstrates a higher coefficient of light absorption within the visible range. Therefore, our findings highlight the tunability of monolayer violet phosphorus properties through the absorption of various fluorine coverages, providing valuable insights for the design and development of novel semiconductor devices based on this material.

Research progress in insulating and thermal conductivity of fluorinated graphene and its polyimide composites

AbstractThe demand for innovative thermal management materials with superior thermal conductivity and electrical insulating properties has significantly increased with the development of portable and flexible electronic gadgets. Fluorinated graphene (FG) has recently attracted the attention of the scientific community because of its exceptional thermal conductivity and electrical insulating qualities. This work aims to provide a detailed analysis of the structure‐property relationships inherent in FG, including both chemical and physical properties, and to explain the FG manufacturing process. Special attention should be paid to a thorough analysis of the thermodynamic conduction mechanism exhibited by FG, including the effects of corrugation size, fluorine coverage, and fluorine atom distribution on its thermal conductivity. The essay also examines in‐depth the most current and cutting‐edge developments addressing the utilisation of FG as a functional filler in composite‐modified polyimide (PI) materials. Furthermore, it has been noted as a crucial component in answering the needs for possible applications by maximising thermal conductivity and mechanical qualities in FG/PI composites through particular FG structural engineering and increased FG‐PI interaction. As a result, these elements serve as the main focus of ongoing research projects, highlighting important directions for development and investigation.

Study of Thermal Conductivity of Fluorinated Graphene

With the rapid development of technology and the advent of the 5G era in recent years, the heat dissipation of electronic devices has received great attention. However, the high intrinsic conductivity of ordinary graphene materials limits their potential applications in electronic packaging materials because of their poor thermal management. The results show that fluorinated graphene has excellent heat resistance, corrosion resistance, and strong wear resistance. It can also play a role in lubrication and is commonly used in high-temperature coatings, wear-resistant lubrication coatings and corrosion-resistant coatings because it does not react easily with other substances. Due to the strong electronegativity of fluorine, fluorinated graphene has strong stability and oxidation resistance at high temperatures. Therefore, in this paper, a fluorinated graphene with high compressibility, high thermal conductivity, and high electrical insulation was developed by hydrothermal method assisted by hydrofluoric acid, and the effect on the thermal conductivity of fluorinated graphene was investigated by changing the fluorine-to-carbon ratio (F/C) by adjusting the hydrofluoric acid content. The structure of fluorinated graphene was characterized by SEM and XRD, which proved to be porous and a customized interconnected graphene network with adjustable fluorine coverage. The prepared fluorinated graphene has good insulating properties with a minimum conductivity of 4×10-7 S cm-1 and a thermal conductivity of 1.254 W m-1 K-1, which has been confirmed by the electrical conductivity test results. Meanwhile, because of the porous structure of graphene fluoride, we prepared epoxy resin/fluorinated graphene nanocomposites by vacuum-assisted infiltration process using epoxy resin as the filler material. This material and fluorinated graphene showed outstanding thermal performance during the typical cooling process. The conclusion shows that graphene fluoride and epoxy resin/fluorinated graphene nanocomposites are promising for electronic packaging.

Fluorination of graphene leads to susceptibility for nanopore formation by highly charged ion impact

The formation of nanopores by highly charged ion impacts on freestanding fluorine-functionalized graphene is demonstrated. The process is driven by potential sputtering, which becomes active by changing the semimetallic property of graphene into a strongly insulating state by fluorination. The interaction of fluorographene with highly charged ions is also studied in terms of charge exchange and kinetic energy loss. A higher number of captured electrons and a larger kinetic energy loss than in pristine graphene are observed, which can be well explained by an increase in the ion neutralization length and in the atomic areal density of the target, respectively. Using a computer code based on a time-dependent scattering potential model, a connection between the efficiency of charge exchange and the fluorine coverage is revealed. Our results suggest a competition of two distinct nanostructure formation processes, leading either to pore formation or fluorine desorption.

Stepwise defluorination of fluorographene: How do the structural features govern the rates of heterogeneous electron transfer?

Partial fluorination of graphene is known to enhance its heterogeneous electron transfer (HET) rates with a significant impact on its application as electrochemical sensors. However, the results had been rather inconclusive on the correlation between the HET rates and the degree of fluorine coverage. We address this using a top-down approach, via the step-wise defluorination of fluorographene using the solvent-assisted, sonochemical method. The results suggest that in addition to the fluorine coverage, the structural and morphological features also have a significant role in governing the HET rates. The reversible limit of HET is attained in the case of glassy carbon electrode modified with partially defluorinated fluorographene with 26% of fluorine coverage studied with the redox probe [Fe(CN)6]4−/3−, and this is correlated to the formation of the unique onion-like interconnected nanocarbon structural features. Electrochemical sensing studies with the prepared fluorographene systems using some important biomarkers like ascorbic acid (AA), uric acid (UA) and dopamine (DA) are also reported.

Thermal degradation behavior of self-assembled monolayer surfactant on silicon substrate

In nanoimprint lithography, a release agent on the mold surface is usually necessary for easy demolding between the mold and the imprinted (thermal) resist. In this work, the thermal stability of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) monolayers is studied using x-ray photoelectron spectroscopy. The FDTS monolayers are deposited on Si (100) substrates via vapor phase reactions. Significant fluorine desorption of the monolayers is observed for samples annealed at 250 and 300 °C in air. The fluorine coverage decreases as a function of annealing time at a given annealing temperature. The desorption is proposed to be dependent on the monolayer packing details and may be influenced by the intermolecular heat transfer. Removal of the CF3 groups is found to be faster than that of the CF2 group as evidenced by the CF2/CF3 peak area ratios that increase with the annealing time. Sessile drop water contact angle and fluorine coverage evolution results show that the estimated useful coating lifetime is 180 min when the samples are annealed at 300 °C and ∼560 min when annealed at 250 °C. The peak position of the binding energy of the F 1s spectral line is related to the monolayer fluorine coverage and it may be a result of the interactions between the molecular chain and the negatively charged silicon substrate. Furthermore, nearly no chain desorption is detected for samples annealed in an inert environment, which may be attributed to the elimination of reactive oxygen and moisture molecules. The thermal degradation behaviors in ambient and inert atmosphere provide useful information for designing a nanoimprint process for the commercial manufacturing of polymeric microstructure and nanostructure.

Interaction between Fluorine and Graphene Vacancy Defects

The results are presented from quantum-chemical simulations of the interaction between F– and FHF– ions and monovacancy and divacancy defects in graphene. The energy characteristics of fluorine chemisorption from ion associates with water molecules are determined. It is shown that the vacancies affect the parameters of chemisorption: the activation energy falls and the heat of adsorption rises, compared to those of an ordered graphene sheet. The relationship between the heat of chemisorption and the degree of fluorine coverage is studied. The characteristics of the reaction between vacancy defects and F–, FHF–, and hydroxonium ions are compared.

Theoretical study of fluorine-induced surface segregation of Cr in non-passivated Ni-based alloys

The effects of fluorine adsorption on the surface segregation behaviors of chromium in nickel-based alloys are systematically investigated by selecting three typical low-index Cr-doped Ni (111), (100), and (110) surfaces based on the first-principles calculations. The Cr doped in the nickel substrate is identified to possess a local trapping effect for fluorine in the full space and contributes to the more exothermic adsorption of fluorine. In turn, the chemisorption of fluorine on the Cr-doped Ni surfaces can dramatically incur the preferential top-surface segregation for Cr from nickel bulk. The increased adsorption energy of fluorine on Cr-doped Ni surfaces compensates for the larger surface energy of Cr relative to Ni. The strong F-Cr bonding from the deeper hybrid state of F 2p-Cr 3d helps to release strain energy caused by atomic size mismatch. At higher fluorine coverage, the driving force for Cr to segregate at nickel surfaces is significantly increased, as the fluorine starts to agglomerate in the vicinity of Cr. Such chemisorption-induced surface abnormal segregation of Cr to the top-most surface plays a critical role in the preferential dealloying of Cr at the early-stage corrosion of non-passivated nickel-based alloys in molten fluoride salts.

Nuclear dynamic effects on electronic properties of functionalized diamondoids

By combining time-dependent density functional theory (TD-DFT) and Franck-Condon (FC) approximation, the effects of nuclear dynamic on the optical gap (OG) of the smallest diamondoids (adamantane and diamantane) when functionalized with three different chemical groups or atom (hydroxyl, amino, fluorine) are studied. The effects of structural relaxation and quantum zero-point on excitation energy are considered. We found that vibrationally-resolved electronic spectra and the amount of the OG renormalizations are modified in small diamondoids by the chemistry of the surface. The lowest unoccupied molecular orbitals (LUMO) do not show significant couplings to vibrations. Functional groups (–OH or NH2) and the fluorine atom show different coupling features. For small diamondoids, HOMO orbitals are dominantly characterized by nonbonding pairs of OH or NH2 groups, which suppress the influence of coupling the cage electrons to nuclear motion through the electronic transition. Destroying the coupling of electrons to nuclear motion is more significant in functionalization with OH group. In contrast, at least for low fluorine coverage, the HOMO orbital is localized inside the cages. Consequently, nuclear dynamics follows the same feature as for pristine diamondoids in fluorinated diamondoids.

Tuning the Basal Plane Functionalization of Two-Dimensional Metal Carbides (MXenes) To Control Hydrogen Evolution Activity

Hydrogen evolution reaction (HER) via electrocatalysis is one method of enabling sustainable production of molecular hydrogen as a clean and promising energy carrier. Previous theoretical and experimental results have shown that some two-dimensional (2D) transition metal carbides (MXenes) can be effective electrocatalysts for the HER, based on the assumption that they are functionalized entirely with oxygen or hydroxyl groups on the basal plane. However, it is known that MXenes can contain other basal plane functionalities, e.g., fluorine, due to the synthesis process, yet the influence of fluorine termination on their HER activity remains unexplored. In this paper, we investigate the role and effect of basal plane functionalization (Tx) on the HER activity of 5 different MXenes using a combination of experimental and theoretical approaches. We first studied Ti3C2Tx produced by different fluorine-containing etchants and found that those with higher fluorine coverage on the basal plane exhibited lower HER activity. We then controllably prepared Mo2CTx with very low basal plane fluorine coverage, achieving a geometric current density of −10 mA cm–2 at 189 mV overpotential in acid. More importantly, our results indicate that the oxygen groups on the basal planes of Mo2CTx are catalytically active toward the HER, unlike in the case of widely studied 2H-phase transition metal dichalcogenides such as MoS2, in which only the edge sites are active. These results pave the way for the rational design of 2D materials for either the HER, when minimal overpotential is desired, or for energy storage, when maximum voltage window is needed.

Tuning the tunneling magnetoresistance by using fluorinated graphene in graphene based magnetic junctions

Spin polarized properties of fluorinated graphene as tunnel barrier with CrO2 as two HMF electrodes are studied using first principle methods based on density functional theory. Fluorinated graphene with different fluorine coverages is explored as tunnel barriers in magnetic tunnel junctions. Density functional computation for different fluorine coverages imply that with increase in fluorine coverages, there is increase in band gap (Eg) of graphene, Eg ∼ 3.466 e V was observed when graphene sheet is fluorine adsorbed on both-side with 100% coverage (CF). The results of CF graphene are compared with C4F (fluorination on one-side of graphene sheet with 25% coverage) and out-of-plane graphene based magnetic tunnel junctions. On comparison of the results it is observed that CF graphene based structure offers high TMR ∼100%, and the transport of carrier is through tunneling as there are no transmission states near Fermi level. This suggests that graphene sheet with both-side fluorination with 100% coverages acts as a perfect insulator and hence a better barrier to the carriers which is due to negligible spin down current (I↓) in both Parallel Configuration (PC) and Antiparallel Configuration (APC).

Partially Fluorinated Graphene: Structural and Electrical Characterization.

Despite the number of existing studies that showcase the promising application of fluorinated graphene in nanoelectronics, the impact of the fluorine bonding nature on the relevant electrical behaviors of graphene devices, especially at low fluorine content, remains to be experimentally explored. Using CF4 as the fluorinating agent, we studied the gradual structural evolution of chemical vapor deposition graphene fluorinated by CF4 plasma at a working pressure of 700 mTorr using Raman and X-ray photoelectron spectroscopy (XPS). After 10 s of fluorination, our XPS analysis revealed a co-presence of covalently and ionically bonded fluorine components; the latter has been determined being a dominant contribution to the observation of two Dirac points in the relevant electrical measurement using graphene field effect transistor devices. Additionally, this ionic C-F component (ionic bonding characteristic charge sharing) is found to be present only at low fluorine content; continuous fluorination led to a complete transition to a covalently bonded C-F structure and a dramatic increase of graphene sheet resistance. Owing to the formation of these various C-F bonding components, our temperature-dependent Raman mapping studies show an inhomogeneous defluorination from annealing temperatures starting at ∼150 °C for low fluorine coverage, whereas fully fluorinated graphene is thermally stable up to ∼300 °C.

Fluorinated graphenes as advanced biosensors - effect of fluorine coverage on electron transfer properties and adsorption of biomolecules.

Graphene derivatives are promising materials for the electrochemical sensing of diverse biomolecules and development of new biosensors owing to their improved electron transfer kinetics compared to pristine graphene. Here, we report complex electrochemical behavior and electrocatalytic performance of variously fluorinated graphene derivatives prepared by reaction of graphene with a nitrogen-fluorine mixture at 2 bars pressure. The fluorine content was simply controlled by varying the reaction time and temperature. The studies revealed that electron transfer kinetics and electrocatalytic activity of CFx strongly depend on the degree of fluorination. The versatility of fluorinated graphene as a biosensor platform was demonstrated by cyclic voltammetry for different biomolecules essential in physiological processes, i.e. NADH, ascorbic acid and dopamine. Importantly, the highest electrochemical performance, even higher than pristine graphene, was obtained for fluorinated graphene with the lowest fluorine content (CF0.084) due to its high conductivity and enhanced adsorption properties combining π-π stacking interaction with graphene regions with hydrogen-bonding interaction with fluorine atoms.

Improvement of work function and hole injection efficiency of graphene anode using CHF3 plasma treatment

We report improvement of hole injection efficiency of a graphene anode by tuning its work function (WF) via surface fluorination. We used chemical vapor deposition to synthesize high-quality graphene sheets and then treated them with CHF3 plasma to induce fluorination. We used x-ray photoelectron spectroscopy to examine the fluorine coverage and the kind of chemical bonds in fluorinated graphene (FG). Also, we used ultraviolet photoelectron spectroscopy to systematically study the changes in the WF and sheet resistance of the FG sheets with varying plasma exposure time (0, 10, 30, 60, 90 s) to find an optimum fluorination condition for hole injection. The WF of graphene sheets was increased by up to 0.74 eV, as a result of the formation of carbon-fluorine bonds that function as negative surface dipoles. We fabricated hole-only devices and conducted dark injection space-charge-limited-current transient measurement; the fluorination greatly increased the hole injection efficiency of graphene anodes (from 0.237 to 0.652). The enhanced hole injection efficiency of FG anodes in our study provides wide opportunities for applications in graphene-based flexible/stretchable organic optoelectronics.

Synthesis of highly luminescent fluorinated graphene quantum dots with tunable fluorine coverage and size

Synthesizing fluorinated graphene quantum dots (FGQDs) has been a great challenge due to the chemically inert C-F bond, and few attempts have been made to prepare this material. In this article, a novel, mild and effective method to controllably prepare FGQDs with tunable fluorine coverage and size by employing commercial fluorinated graphite is developed. This method avoids the complex procedures of preparing fluorinated graphene and use of toxic gases, and can be realized under mild conditions. Moreover, the obtained FGQDs possess relatively uniform size, enjoy good solubility and stability in water and exhibit highly bright blue luminescence.

To lose is to gain: Effective synthesis of water-soluble graphene fluoroxide quantum dots by sacrificing certain fluorine atoms from exfoliated fluorinated graphene

Water-soluble and blue luminescent graphene fluoroxide quantum dots (GFOQDs) with tunable fluorine coverage and size were effectively synthesized from exfoliated fluorinated graphene (FG) by sacrificing certain fluorine to improved solubility and reaction activity. Morphology investigation indicates that the obtained GFOQDs possess narrow size distribution and the average size is 2.5–3.5nm. Chemical composition analysis confirms that besides C–F covalent bonds, C–O bonds in the forms of hydroxyl and carbonyl co-exist on the structure of GFOQDs. Moreover, photoluminescence performance research considering the surface state and size has also been conducted, and as anticipated in carbon-based quantum dots the GFOQDs exhibit excitation wavelength-dependent properties. Additionally, rather different from other graphene quantum dots (GQDs) that are often susceptible to pH without additional surface passivation, the GFOQDs themselves are poised to resist pH effects and display stable luminescence in both acid and alkali conditions. These results indicate that our method not only opens up a new avenue to prepare GQDs decorated with fluorine and oxygen, but also can find practical applications in novel GQDs-based devices that require water solubility while keep chemical stability and resistance against pH.

First principles study of fluorine substitution on two-dimensional germanane

Using first principles calculations with hybrid exchange-correlation functional, we investigate the electronic structure of germanane and fluorinated germanane. We find that the band gap of fluorinated germanane decreases with the increasing of fluorine coverage, regardless of fluorine distribution on the surface. Our results indicate that chemical functionalization could be used easily to control the band gap of the germanane from 1.55 eV (pure germanane) to 0.17 eV (fully fluorinated germanane). It also shows good potentiality as a photocatalytic material for water splitting, as well as a two-dimensional functional material for optoelectronic and sensing applications.

Fluorination of Graphene: A Spectroscopic and Microscopic Study

Since the advent of graphene, there has been intense interest in exploring the possibility of incorporating fluorinated graphene (FG), an ultrathin insulator, into graphene electronics as barriers, gate dielectrics, and optoelectronic elements. Here we report on the synthesis of FG from single-layer graphene sheets grown by chemical vapor deposition (CVD) using CF4 plasma. We examine its properties systematically via microscopic and spectroscopic probes. Our studies show that, by controlling the conditions of the plasma, FG of varying fluorine coverage can be produced; however, the resulting material contains a mixture of CFx (x = 1-3) bonds. Existing grain boundaries and lattice defects of CVD graphene play an important role in controlling its rate of fluorination and the damage of the sheet. Combining topography and current mapping, we demonstrate that the spatial distribution of fluorine on CVD graphene is highly inhomogeneous, where multilayer islands and structural features such as folds, wrinkles, and ripples are less fluorinated and consequently form a conductive network through which charge transport occurs. It is the properties of this network that manifest in the electrical transport of FG sheets. Our experiments reveal the many challenges of deriving electronics-quality FG from current CVD graphene while at the same time point to the possible solutions and potential of FG in graphene electronics and optoelectronics.

Deposition of Highly Porous Nanocrystalline Platinum on Functionalized Substrates Through Fluorine-Induced Decomposition of Pt(PF3 )4 Adsorbates

Nanocrystalline platinum is synthesized at room temperature by co-injecting Pt(PF3)4 and XeF2 vapors onto solid supports in vacuum. The Pt nucleation time scales with chemisorbed fluorine coverage, which is controlled by pre-dosing supports with XeF2, and by optional electron or ion beam irradiation under flowing XeF2. The latter is used to increase the chemisorbed fluorine coverage and localize the Pt growth process.

Layers and tubes of fluorographene C4F: Stability, structural and electronic properties from DFTB calculations

By means of the DFTB band structure calculations we have explored the layers’ isomerism of fluorographene C4F. The relative stability, structural and electronic properties of the C4F layers and nanotubes have been revealed depending on the possible types of fluorine coverage: single-sided, double-sided or so-called non-uniform variants. Our main finding is that the aforementioned types of fluorine coverage are crucial for the morphology of these materials. At the non-uniform or single-sided coverage types the C4F structures aspire to the spontaneous folding in order to minimize their surface tension.