Strong P-doping Research Articles

Evidence for Trans-Oligoene Chain Formation in Graphene Induced by Iodine.

Functionalization of pristine graphene by hydrogen and fluorine is well studied, resulting in graphane and fluorographene structures. In contrast, functionalization of pristine graphene with iodine has not been reported. Here, the functionalization of graphene with iodine using photochemical activation is presented, which is thermally reversible at 400°C. Additional dispersive dominant Raman modes that are probed by resonance Raman spectroscopy are observed. Additionally, iodinated graphene is probed by Kelvin probe force microscopy and by transport measurements showing p-doping surpassing non-covalent iodine doping by charge transfer-complex formation. The emergent Raman modes combined with strong p-doping indicate that iodine functionalization is distinct from simple iodine doping. A reaction mechanism based on these findings is proposed, identifying the large size of iodine atoms as the probable cause governing regiochemically controlled addition due to steric hinderance of reactive sites. The modification of the electronic structure is explained by the confinement of 1D trans-oligoene chains between sp3-defects. These results demonstrate the uniqueness of iodine reactivity toward graphene and the modification of the electronic structure of iodinated graphene, highlighting its dependence on the spatial arrangement of substituents.

Highly doped graphene on ion-exchanged glass

Engineering the doping level in graphene is essential to realizing functional electronic and optoelectronic devices. While achieving strong p-doping is relatively straightforward, electrostatic or chemical approaches to negatively dope graphene have yielded electron densities (ns ) of −9.5 × 1012 cm−2 or below. In this work, we demonstrate very high ns (−1013 to −1014 cm−2) in graphene, on an ion-exchanged glass substrate, which is widely used in touch screen displays (e.g. smart phones). Moreover, the proposed method, which is easy to implement and scalable, leads to relatively stable graphene doping, with about a 40% increase in sheet resistance over 5 months at ambient conditions.

Enabling Quick Response to Nitrogen Dioxide at Room Temperature and Limit of Detection to Ppb Level by Heavily n-Doped Graphene Hybrid Transistor.

Sensitive detection of nitrogen dioxide (NO2) is of significance in many areas for health and environmental protections. In this work, we developed an efficient NO2 sensor that can respond within seconds at room temperature, and the limit of detection (LOD) is as low as 100 ppb. Coating cyano-substituted poly(p-phenylene vinylene) (CN-PPV) films on graphene (G) layers can dope G sheets effectively to a heavy n state. The influences of solution concentrations and annealing temperatures on the n-doping effect were investigated in detail. The CN-PPV-G transistors fabricated with the optimized parameters demonstrate active sensing abilities toward NO2. The n-doping state of CN-PPV-G is reduced dramatically by NO2, which is a strong p-doping compound. Upon exposure to 25 ppm of NO2, our CN-PPV-G sensors react in 10 s, indicating it is almost an immediate response. LOD is determined as low as 100 ppb. The ultrahigh responding speed and low LOD are not affected in dry air. Furthermore, cycling use of our sensors can be realized through simple annealing. The superior features shown by our CN-PPV-G sensors are highly desired in the applications of monitoring the level of NO2 in situ and setting immediate alarms. Our results also suggest that transfer curves of transistors can react very promptly to the stimulus of target gas and, thus, are very promising in the development of fast-response sensing devices although the response values may not reach maximum as a tradeoff.

Highly efficient SWCNT/GaAs van der Waals heterojunction solar cells enhanced by Nafion doping

The atomic-level interface and strong interfacial interaction enable van der Waals (vdW) heterostructures to fabricate innovative photovoltaic devices by combing single-walled carbon nanotubes (SWCNTs) with bulk semiconductors, in which the device performance can be enhanced significantly using outstanding photoelectric property of SWCNTs. Herein, we reported a facile method to fabricate SWCNT/GaAs vdW heterojunction solar cell by transferring p-type (6,5)-enriched SWCNT film onto the n-type GaAs substrate, a power conversion efficiency (PCE) of 7.23 % has been achieved without any other treatment. Furthermore, the PCE value can be enhanced to 11.24 % by introducing Nafion as a highly effective dopant with a short-circuit current density of 24.70 mA/cm2, an open-circuit voltage of 0.64 V and a fill factor of 0.71. Our results reveal that the introduction of Nafion not only induces a stronger and closer contact at the interface between SWCNTs and GaAs, but also shows strong p-doping effect due to the existence of sulfonic acid functional groups. This p-doping effect of Nafion increases the work function of SWCNTs and subsequently enlarges the barrier height at the heterojunction for more efficient carrier separation. In addition, Nafion can also serve as an antireflection and passivation layer, reducing the incident light loss and the carrier recombination rate. These findings suggest that Nafion could be used to improve the performance of SWCNT-based vdW heterojunction devices.

Effective hole conductivity in nitrogen-doped CVD-graphene by singlet oxygen treatment under photoactivation conditions

Nitrogen substitutional doping in the π-basal plane of graphene has been used to modulate the material properties and in particular the transition from hole to electron conduction, thus enlarging the field of potential applications. Depending on the doping procedure, nitrogen moieties mainly include graphitic-N, combined with pyrrolic-N and pyridinic-N. However, pyridine and pyrrole configurations of nitrogen are predominantly introduced in monolayer graphene:N lattice as prepared by CVD. In this study, we investigate the possibility of employing pyridinic-nitrogen as a reactive site as well as activate a reactive center at the adjacent carbon atoms in the functionalized C–N bonds, for additional post reaction like oxidation. Furthermore, the photocatalytic activity of the graphene:N surface in the production of singlet oxygen (1O2) is fully exploited for the oxidation of the graphene basal plane with the formation of pyridine N-oxide and pyridone structures, both having zwitterion forms with a strong p-doping effect. A sheet resistance value as low as 100 Ω/□ is reported for a 3-layer stacked graphene:N film.

Surface charge transfer doping of graphene using a strong molecular dopant CN6-CP

Surface charge transfer doping of graphene plays an important role in graphene-based electronics due to its simplicity, high doping efficiency, and easy-controllability. Here, we demonstrate the effective surface charge transfer hole doping of graphene by using a strong p-type molecular dopant hexacyano-trimethylene-cyclopropane (CN6-CP). The CN6-CP exhibits a very high intrinsic work function of 6.37 eV, which facilitates remarkable electron transfer from graphene to CN6-CP as revealed by in situ photoelectron spectroscopy investigations. Consequently, hole accumulation appears in the graphene layer at the direct contact with CN6-CP. As evidenced by Hall effect measurements, the areal hole density of graphene significantly increased from 8.3 × 1012 cm−2 to 2.21 × 1013 cm−2 upon 6 nm CN6-CP evaporation. The CN6-CP acceptor with strong p-doping effect has great implications for both graphene-based and organic electronics.

Reversing the Polarity of MoS2 with PTFE.

Pristine monolayer molybdenum disulfide (MoS2) demonstrates predominant and persistent n-type semiconducting polarity due to the natural sulfur vacancy, which hinders its electronic and optoelectronic applications in the rich bipolarity area of semiconductors. Current doping strategies in single-layer MoS2 are either too mild to reverse the heavily n-doped polarity or too volatile to create a robust electronic device meeting the requirements of both a long lifetime and compatibility for mass production. Herein, we demonstrate that MoS2 can be transferred onto polytetrafluoroethylene (PTFE), one of the most electronegative substrates. After transfer, the MoS2 photoluminescence exhibits an obvious blueshift from 1.83 to 1.89 eV and a prolonged lifetime, from 0.13 to 3.19 ns. The Fermi level of MoS2 experiences a remarkable 510 meV decrease, transforming its electronic structure into that of a hole-rich p-type semiconductor. Our work reveals a strong p-doping effect and charge transfer between MoS2 and PTFE, shedding light on a new nonvolatile strategy to fabricate p-type MoS2 devices.

Sandwich-Doping for a Large Schottky Barrier and Long-Term Stability in Graphene/Silicon Schottky Junction Solar Cells.

Doping is an effective method for controlling the electrical properties and work function of graphene which can improve the power conversion efficiency of graphene-based Schottky junction solar cells (SJSCs). However, in previous approaches, the stability of chemical doping decreased over time due to the decomposition of dopants on the surface of graphene under ambient conditions. Here, we report an efficient and strong p-doping by simple sandwich doping on both the top and bottom surfaces of graphene. We confirmed that the work function of sandwich-doped graphene increased by 0.61 eV and its sheet resistance decreased by 305.8 Ω/sq, compared to those of the pristine graphene. Therefore, the graphene-silicon SJSCs that used sandwich-doped graphene had a power conversion efficiency of 10.02%, which was 334% higher than that (2.998%) of SJSCs that used pristine graphene. The sandwich-doped graphene-based silicon SJSCs had excellent long-term stability over 45 days without additional encapsulation.

Electronic structure of graphene/MoO3/CuPc interfaces

The interfacial electronic structures of graphene/copper phthalocyanine (CuPc) modified with MoO3 interlayers were investigated using x-ray and ultraviolet photoelectron spectroscopy. Strong p-doping effect has been observed on graphene due to deposition of MoO3 films, causing formation of surface dipole (eD) and shift of graphene Dirac point, and consequently altering the charge transfer at graphene/CuPc interface. The ionization energy of CuPc on the graphene/MoO3 substrate is about 4.97 eV. The interfacial energy level alignments are estimated based on the experimental photoelectron spectroscopy data. Finally, the present experimental data indicate that the height of the electron and hole injection barriers from CuPc to graphene/MoO3 are 1.98 and 0.32 eV, respectively.

Photoemission investigation of oxygen intercalated epitaxial graphene on Ru(0001)

We study the formation of epitaxial graphene on Ru(0001) using fast x-ray photoelectron spectroscopy during the growth process. The assignment of different C 1s and Ru 3d core level components and their evolution during the growth process gives a detailed insight into the graphene formation and the strongly varying graphene-Ru interaction strength within the large moiré unit cell. Subsequent intercalation of oxygen can be achieved at elevated temperature and the core level spectra show a conversion of the strongly corrugated to quasi free-standing graphene, characterised by a single narrow C 1s component. This conversion and the accompanying flattening of the graphene layer is also confirmed by x-ray photoelectron diffraction. The effect of oxygen intercalation on the electronic structure is studied using angle-resolved photoemission of the valence band states. For graphene/Ru(0001), the strong graphene-substrate hybridisation disrupts the π-band dispersion but oxygen intercalation fully restores the π-band with a strong p-doping that shifts the Dirac point 785 meV above the Fermi level. The doping of the system is highly tunable, as the additional exposure to rubidium can convert the carrier filling to n-type with the Dirac point 970 meV below the Fermi level.

Hybrid Organic/Inorganic Perovskite-Polymer Nanocomposites: Toward the Enhancement of Structural and Electrical Properties.

Hybrid organic/inorganic perovskite nanoparticles (NPs) have garnered remarkable research attention because of their promising photophysical properties. New and interesting properties emerge after combining perovskite NPs with semiconducting materials. Here, we report the synthesis and investigation of a composite material obtained by mixing CH3NH3PbBr3 nanocrystals with the semiconducting polymer poly(3-hexylthiophene) (P3HT). By the combination of structural techniques and optical and magnetic spectroscopies we observed multiple effects of the perovskite NPs on the P3HT: (i) an enlargement of P3HT crystalline domains, (ii) a strong p-doping of the P3HT, and (iii) an enhancement of interchain order typical of H-aggregates. These observations open a new avenue toward innovative perovskite NP-based applications.

Probing Single Vacancies in Black Phosphorus at the Atomic Level.

Utilizing a combination of low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and electronic structure calculations, we characterize the structural and electronic properties of single atomic vacancies within several monolayers of the surface of black phosphorus. We illustrate, with experimental analysis and tight-binding calculations, that we can depth profile these vacancies and assign them to specific sublattices within the unit cell. Measurements reveal that the single vacancies exhibit strongly anisotropic and highly delocalized charge density, laterally extended up to 20 atomic unit cells. The vacancies are then studied with STS, which reveals in-gap resonance states near the valence band edge and a strong p-doping of the bulk black phosphorus crystal. Finally, quasiparticle interference generated near these vacancies enables the direct visualization of the anisotropic band structure of black phosphorus.

Thermal conductivity of silicon carbide composites with highly oriented graphene nanoplatelets

Dense silicon carbide (SiC)-based composites containing up to 20vol.% of graphene nanoplatelets (GNPs) were manufactured via a liquid-phase Spark Plasma Sintering (SPS) process, employing Y2O3 and Al2O3 as sintering aids. The applied uniaxial pressure during SPS produced a preferential orientation of the GNPs, with the ab-plane perpendicular to the pressing axis, thus bringing about high thermal anisotropy. Thermal diffusivity/conductivity were measured as a function of the GNPs content, being enhanced by ∼30% in the perpendicular direction to the SPS pressing axis, whereas a moderate decrease was observed in the parallel direction owing to the much lower intrinsic conductivity of the aligned GNPs in the c-axis, with no significant contribution of the thermal resistance of the GNP-SiC interfaces. A non-monotonical temperature dependence for the thermal conductivity was detected in composites containing <10vol.% GNPs, explained by the strong p-doping of these SiC ceramics.

Structural and electronic characterization of graphene grown by chemical vapor deposition and transferred onto sapphire

We present a combination of magnetotransport and local probe measurements on graphene grown by chemical vapor deposition on copper foil and subsequently transferred onto a sapphire substrate. A rather strong p-doping is observed (∼9×1012cm−2) together with quite low carrier mobility (∼1350cm2/Vs). Atomic force and tunneling imaging performed on the transport devices reveals the presence of contaminants between sapphire and graphene, explaining the limited performance of our devices. The transferred graphene displays ridges similar to those observed whilst graphene is still on the copper foil. We show that, on sapphire, these ridges are made of different thicknesses of the contamination layer and that, contrary to what was reported for hBN or certain transition metal dichalcogenides, no self-cleansing process of the sapphire substrate is observed.

Spontaneous and strong multi-layer graphene n-doping on soda-lime glass and its application in graphene-semiconductor junctions.

Scalable and low-cost doping of graphene could improve technologies in a wide range of fields such as microelectronics, optoelectronics, and energy storage. While achieving strong p-doping is relatively straightforward, non-electrostatic approaches to n-dope graphene, such as chemical doping, have yielded electron densities of 9.5 × 1012 e/cm2 or below. Furthermore, chemical doping is susceptible to degradation and can adversely affect intrinsic graphene’s properties. Here we demonstrate strong (1.33 × 1013 e/cm2), robust, and spontaneous graphene n-doping on a soda-lime-glass substrate via surface-transfer doping from Na without any external chemical, high-temperature, or vacuum processes. Remarkably, the n-doping reaches 2.11 × 1013 e/cm2 when graphene is transferred onto a p-type copper indium gallium diselenide (CIGS) semiconductor that itself has been deposited onto soda-lime-glass, via surface-transfer doping from Na atoms that diffuse to the CIGS surface. Using this effect, we demonstrate an n-graphene/p-semiconductor Schottky junction with ideality factor of 1.21 and strong photo-response. The ability to achieve strong and persistent graphene n-doping on low-cost, industry-standard materials paves the way toward an entirely new class of graphene-based devices such as photodetectors, photovoltaics, sensors, batteries, and supercapacitors.

Efficient p-type doping in graphene via monolayer manganese-oxide nanoparticles decoration

We demonstrated that manganese-oxide nanoparticles (Mn3O4 NPs) acted as an efficient p-type dopant for graphene. Uniformly-sized were Mn3O4 NPs (~5.5 nm in diameter) were synthesized by using a thermal decomposition method and were decorated on single-layer graphene by using an ex-situ method. The morphology and the bonding status of the NP-decorated graphene were characterized by using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Mn3O4 clusters with a lateral size of 10–50 nm and a monolayer thickness were decorated on the graphene. The strong p-doping effect was clearly evident in the Raman spectroscopy data where the positions of the G-band and the 2D-band of graphene progressively upshifted. The p-doping effect of Mn3O4 NP decoration was also compared with that induced by an annealing treatment of graphene. This approach enabled us to efficiently induce the p-type property in graphene and exhibited a potential for developing high-performance graphene-based electronic devices.

Doping graphene with an atomically thin two dimensional molecular layer.

Atomically thin and chemically versatile GO sheets are used as p-type dopants of CVD-graphene. This method enables the strong, stable, large-scale, low-temperature, and controllable p-doping of graphene with preserved charge mobility, intrinsic roughness, and transmittance.

Enhanced thermoelectric figure of merit in strained Tl-doped Bi2Se3

We explain recent experimental findings on Tl-doped Bi2Se3 by determining the electronic and transport properties by first-principles calculations and semi-classical Boltzmann theory. Though Tl-doping introduces a momentum-dependent spin-orbit splitting, the effective mass of the carriers is essentially not modified, while the band gap is reduced. Tl is found to be exceptional in this respect as other dopants modify the dispersion, which compromises thermoelectricity. Moreover, we demonstrate that only after Tl-doping strain becomes an efficient tool for enhancing the thermoelectric performance. A high figure of merit of 0.86 is obtained for strong p-doping (7 × 1020 cm−3, maximal power factor) at 500 K under 2% tensile strain.

Formation of p–n junction with stable p-doping in graphene field effect transistors using deep UV irradiation

We demonstrate the modification of the electronic properties of single layer chemical vapor deposition (CVD)-grown graphene by deep ultraviolet (DUV) light irradiation. The shift in the G and 2D bands in Raman spectra towards higher wavenumber suggests p-doping in graphene field effect transistors (FETs). In the transport measurements, the Dirac point is shifted towards positive gate voltage with increasing DUV light exposure time, revealing the strong p-doping effect without a large resistance increase. The doping is found to be stable in graphene devices, with a slight change in mobilities. We also constructed a p–n junction by DUV light exposure on selected regions of graphene, and investigated it with gate voltage dependent resistivity measurements and current–voltage characteristics.

Chemical modification of graphene characterized by Raman and transport experiments

A chemical approach to modify the electronic transport of graphene is investigated by detailed transport and Raman spectroscopy measurements on Hall bar shaped samples. The functionalization of graphene with nitrobenzene diazonium ions results in a strong p-doping of the graphene samples and only slightly lower mobilities. Comparing Raman and transport data taken after each functionalization step allowed the conclusion that two preferential reactions take place on the graphene surface. In the beginning a few nitrobenzene molecules are directly attached to the graphene atoms creating defects. Afterwards these act as seeds for a polymer like growth not directly connected to the graphene atoms. The effects of solvents were excluded by thorough control measurements.