Photochemical Doping Research Articles

Colloidal Multi-Dot Nanorods.

Colloidal nanorod heterostructures consisting of multiple quantum dots within a nanorod (n-DNRs, where n is the number of quantum dots within a nanorod) are synthesized with alternating segments of CdSe "dot" and CdS "rod" via solution heteroepitaxy. The reaction temperature, time dependent ripening, and asymmetry of the wurtzite lattice and the resulting anisotropy of surface ligand steric hindrance are exploited to vary the morphology of the growing quantum dot segments. The alternating CdSe and CdS growth steps can be easily repeated to increment the dot number in unidirectional or bidirectional growth regimes. As an initial exploration of electron occupation effects on their optical properties, asymmetric 2-DNRs consisting of two dots of different lengths and diameters are synthesized and are shown to exhibit a change in color and an unusual photoluminescence quantum yield increase upon photochemical doping.

Band-Edge Energy Levels of Dynamic Excitons in Cube-Shaped CdSe/CdS Core/Shell Nanocrystals.

An electron-hole pair in a cube-shaped CdSe/CdS core/shell nanocrystal exists in the form of dynamic excitons across the strongly and weakly confined regimes under ambient temperatures. Photochemical doping is applied to distinguish the band-edge electron and hole levels, confirming an effective mass model with universal constants. Reduction of the optical bandgap upon epitaxy of the CdS shells is caused by lowering the band-edge electron level and barely affecting the band-edge hole level. Similar shifts of the electron levels, yet retaining the hole levels, can switch the order in energy of the three lowest-energy transitions. Thermal distribution of 1-4 electrons among the two thermally accessible electron levels follows number-counting statistics, instead of Fermi-Dirac distribution.

Enhanced pseudocapacitance of Ti3C2Tx MXene by UV photochemical doping

This work presents an ultraviolet-assisted photochemical doping strategy for realizing the modification of Ti3C2Tx MXene. In this strategy, nitrogen atoms are easily doped into Ti3C2Tx MXene, and harmful fluorine-containing terminal groups are effectively removed from the Ti3C2Tx MXene under UV light irradiation. The results further show that the nitrogen doping level is ∼2.99 at. %, and the interlayer spacing of Ti3C2Tx MXene increases from 1.271 to 1.363 nm after the nitrogen doping. The nitrogen-doped Ti3C2Tx MXene exhibits a higher specific capacitance of 491 F g−1 (1176 F cm−3) at 2 mV s−1 than raw Ti3C2Tx MXene (292 F g−1) when used as a supercapacitor electrode material. Moreover, a nitrogen-doped Ti3C2Tx quasi-solid-state supercapacitor device can yield an energy density of 14.9 Wh kg−1 at 750 W kg−1. Therefore, this work affords a simple and highly versatile route for fabricating high-performance MXene materials for supercapacitors.

Colloidal n-Doped CdSe and CdSe/ZnS Nanoplatelets.

Colloidal semiconductor nanoplatelets (NPLs) are chemical versions of well-studied quantum wells (QWs). For QWs, gating and carrier doping are standard tools to manipulate their optical, electric, or magnetic properties. It would be highly desirable to use pure chemical methods to dope extra charge carriers into free-standing colloidal NPLs to achieve a similar level of manipulation. Here we report colloidal n-doped CdSe and CdSe/ZnS NPLs achieved through a photochemical doping method. The extra electrons doped into the conduction band edges are evidenced by exciton absorption bleaches recoverable through dedoping and the appearance of new intersub-band transitions in the near-infrared. A high surface ligand coverage is the key to successful doping; otherwise, the doped electrons can be depleted likely by unpassivated surface cations. Large trion binding energies of 20-30 meV are found for the n-doped CdSe NPLs, which, in contrast, are reduced by 1 order of magnitude in CdSe/ZnS core/shell NPLs due to dielectric screening. Furthermore, we identify a long-lived negative trion with a lifetime of 1.5-1.6 ns that is likely dominated by radiative recombination.

Tuning the valley polarization in WS2 monolayers via control of active defect sites induced by photochemical doping

The sufficient control of the carrier density of a single layer WS2 (1L-WS2) has been realized by the pulsed laser irradiation doping technique. Chlorine atoms are incorporated on the surface of the atomically thin lattice in a precursor gas atmosphere. In this work, we demonstrate spin-valley polarization tunability by more than 40% in 1L-WS2 on hBN via photochlorination. Polarization photoluminescence spectroscopy was performed in the temperature range from 4 K to 300 K. The decrease in circular polarization after the photochlorination treatment is attributed to the significant reduction of the active defect sites in 1L-WS2 and, consequently, to the increase in the non-radiative exciton lifetime. Ultrafast time-resolved transient absorption spectroscopy measurements support our findings. The above results indicate a useful approach of controlling the density of the active defect sites and the valley polarized light emission in doped monolayer crystal lattices.

Photoexcitation triggering via semiconductor Graphene Quantum Dots by photochemical doping with Curcumin versus perio-pathogens mixed biofilms

Recently, antimicrobial photodynamic therapy (aPDT) as an alternative treatment modality has been used adjunctively in the treatment of periodontitis and peri-implantitis. Photosensitizing agents in the form of nanoparticles have been designed for improving the efficiency of aPTD. Graphene quantum dots are a special type of nanocrystals that can promote aPDT when coupled with curcumin (Cur). The main objective of the present study was to investigate the effects of photoexcited GQD-Cur on the metabolic activity of perio-pathogen mixed biofilms. GQD-Cur was synthesized and characterized by scanning electron microscopy (SEM), dynamic light scattering (DLS), fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible spectrometry (UV-Vis), and X-ray diffraction (XRD). The cell cytotoxicity effect of GQD-Cur was evaluated on primary human gingival fibroblast (HuGu) cells. Perio-pathogen mixed biofilms including Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, and Prevotella intermedia photosensitized with GQD doped with Cur were irradiated with a blue LED at a wavelength of 435 ± 20 nm for 1 min, and then bacterial viability measurements were performed. The antimicrobial susceptibility profile, biofilm formation ability, amount of reactive oxygen species (ROS) released, and variations of gene expressions involved in biofilm formation were assessed. The SEM, DLS, FTIR, UV-Vis spectrometry, and XRD pattern confirmed that GQD-Cur was synthesized successfully. According to the results, GQD-Cur exhibited no cytotoxicity against HuGu cells. Photoexcited GQD-Cur resulted in a significant reduction in cell viability (93%) and biofilm formation capacity (76%) of peri-pathogens compared to the control group (P < 0.05). According to the results, a significant concentration-dependent increase in the ROS generation was observed in perio-pathogens mixed cells treated with different doses of GQD-Cur-aPDT. Moreover, rcpA, fimA, and inpA gene expression profiles were downregulated by 8.1-, 9.6-, and 11.8-folds, respectively. Based on the results, photoexcited GQD-Cur have a high potency of perio-pathogens suppression in planktonic and biofilm forms and downregulation of the biofilm genes expression pattern was exploited as a nanoscale-based platform for periodontitis.

Structural improvement for stabilization of photochemical hydrogen doping in ZnO thin film

ZnO, a cost-effective metal-oxide semiconductor, has received many attentions for sensing and optoelectronic applications. To improve its properties for developing practical applications, our study on investigating effects of H-doping with UV photochemistry on ZnO thin films, especially electrical property, was presented with the use of a thin film transistor (TFT) platform. Here, ZnO thin films, used as the active channel, were deposited by radio frequency plasma sputtering at room temperature. For an investigation of structural effect, post-deposition annealing (PDA) was carried out before UV irradiation used to introduce H species. By combining these two processes, a significant improvement of the electrical property of ZnO based TFTs, due to the crystallization enhancement along with an increase of doping level, was observed and discussed in detail. Overall, our approach was simple but effective to tune properties of oxide semiconductors.

Observation of positive and negative trions in organic-inorganic hybrid perovskite nanocrystals

Lead halide perovskite nanocrystals are promising light emitting materials with high photoluminescence quantum yields and widely tunable emission wavelengths. Their optical responses are significantly influenced by efficient nonradiative Auger recombination of trions (charged excitons) and biexcitons. Here, we report the observation of positive and negative trions in formamidinium lead bromide nanocrystals and discuss their role in the photoluminescence dynamics. It is found that the addition of copper thiocyanate causes an additional intermediate state by photochemical doping. We clarify that as-prepared nanocrystals show a two-state blinking between neutral excitons and positive trions, while the postsynthesis treated nanocrystals exhibit blinking between all three states, including negative trions. We confirmed that the biexciton Auger rate evaluated from femtosecond transient absorption measurements can be expressed as the sum of the Auger rates of positive and negative trions obtained by single-dot spectroscopy.

Self-assembled formation of conjugated 3D reduced graphene oxide-wrapped helical CNTs nanostructure and nitrogen-doped using photochemical doping for high-performance supercapacitor electrodes

Interconnected three-dimension (3D) networks of novel helical carbon nanotubes (HCNTs) wrapped with reduced graphene oxide nanosheets (HCNTs/rGO) are successfully fabricated via a facile solution of self-assembly method, as well as a robust process for the simultaneous reduction and high N-doping of HCNTs/rGO composites (N-HCNTs/rGO) by photoreduction under NH3 atmosphere. The as-prepared N-HCNTs/rGO are directly employed as binder-free supercapacitor electrodes, and exhibit a highly conductive 3D-interconnected structure (5.85 S cm−1), large surface area (528.9 m2 g−1), low internal resistance (0.5 Ω), and good wettability. As a result, N-HCNTs/rGO show high specific capacitance (368 F g−1), high energy density (12.8 Wh kg−1), and cycling stability (90.7% retention at 1 A g−1 for 5000 cycles) in two-electrode systems. Moreover, the 3D N-HCNTs/rGO hybrid networks exhibit enhanced electrochemical performance in supercapacitors, which combine the synergistic effects of the two carbon nanostructures, enhanced wettability, low internal resistance, and improved ion-diffusion ability, together with the large surface areas of 3D hybrid networks and high-level N-doping. The as-synthesized composite is a potential candidate for flexible and binder-free electrodes for high-performance supercapacitors.

Tuning the Photoluminescence of Graphene Quantum Dots by Photochemical Doping with Nitrogen.

Nitrogen-doped graphene quantum dots (NGQDs) were synthesized by irradiating graphene quantum dots (GQDs) in an NH3 atmosphere. The photoluminescence (PL) properties of the GQDs and the NGQDs samples were investigated. Compared with GQDs, a clear PL blue-shift of NGQDs could be achieved by regulating the irradiating time. The NGQDs obtained by irradiation of GQDs for 70 min had a high N content of 15.34 at % and a PL blue-shift of about 47 nm. This may be due to the fact that photochemical doping of GQDs with nitrogen can significantly enhance the contents of pyridine-like nitrogen, and also effectively decrease the contents of oxygen functional groups of NGQDs, thus leading to the observed obvious PL blue-shift.

Effect of photochemical hydrogen doping on the electrical properties of ZnO thin-film transistors

We examined the effect of photochemical hydrogen doping on the performance of ZnO thin-film transistors (TFTs). H atoms were doped into the ZnO active layers (ZnO:H) by irradiation at two different ultraviolet (UV) wavelengths, 185 and 254 nm, for 0, 2, 4, and 6 h. Then, the ZnO surface before and after H doping was analyzed using X-ray photoelectron spectroscopy and X-ray diffraction. The ZnO surface exhibited metal–OH bonding states and the oxygen vacancy increased with the UV irradiation time. In addition, the surface roughness of the ZnO thin film decreased from 1.634 to 1.385 nm because of the increasing hydrophilicity as the oxygen vacancy increased, reducing the trap sites and electron scattering at the channel/insulator interface, thus improving the TFT performance. Among the samples, the ZnO:H TFTs with 6 h of UV irradiation time exhibited the best performance, the saturation mobility increased from 1.56 to 14.2 cm2/Vs, the on/off ratio increased from 105 to 1.1 × 106, and the threshold voltage decreased from 5.42 to 2 V compared to the ZnO TFTs with no UV irradiation.

Graphene oxide quantum dot-derived nitrogen-enriched hybrid graphene nanosheets by simple photochemical doping for high-performance supercapacitors

Nitrogen-enriched graphene was fabricated via a facile strategy. Graphene oxide (GO) nanosheets and graphene oxide quantum dots (GQDs) were used as a structure-directing agent and in situ activating agent, respectively, after photoreduction under NH3 atmosphere. The combination of photoreduction and NH3 not only reduced GO and GQD composites (GO/GQDs) within a shorter duration but also doped a high level of nitrogen on the composites (NrGO/GQDs). The nitrogen content of NrGO/GQDs reached as high as 18.86 at% within 5min of irradiation. Benefiting from the nitrogen-enriched GO/GQDs hybrid structure, GQDs effectively prevent the agglomeration of GO sheets and increased the numbers of ion channels in the material. Meanwhile, the high levels of nitrogen improved electrical conductivity and strengthened the binding energy between GQD and GO sheets. Compared with reduced GO and low nitrogen-doped reduced GO, NrGO/GQD electrodes exhibited better electrochemical characteristics with a high specific capacitance of 344Fg−1 at a current density of 0.25Ag−1. Moreover, the NrGO/GQD electrodes exhibited 82% capacitance retention after 3000 cycles at a current density of 0.8Ag−1 in 6M KOH electrolyte. More importantly, the NrGO/GQD electrodes deliver a high energy density of 43Whkg−1 at a power density of 417Wkg−1 in 1M Li2SO4 electrolyte. The nitrogen-doped graphene and corresponding supercapacitor presented in this study are novel materials with potential applications in advanced energy storage systems.

Reversible Photochemical Control of Doping Levels in Supported Graphene

Controlling the type and density of charge carriers in graphene is vital for a wide range of applications of this material in electronics and optoelectronics. To date, chemical doping and electrostatic gating have served as the two most established means to manipulate the carrier density in graphene. Although highly effective, these two approaches require sophisticated graphene growth or complex device fabrication processes to achieve both the desired nature and the doping densities with generally limited dynamic tunability and spatial control. Here, we report a convenient and tunable optical approach to tune the steady-state carrier density and Fermi energy in graphene by photochemically controlling the concentration of adsorbed molecular O2, a p-dopant in graphene, using femtosecond pulsed laser irradiation in the UV range. As an all-optical approach, it allows spatial control over doping levels. Combined terahertz (THz) spectroscopy and electrical device measurements reveal that the Fermi level in laser-illuminated graphene can be controllably and reversibly tuned between p- and n-type in a large range (over ∼600 meV from −420 to +180 meV) by readily tuning the peak intensity and the duration of the laser irradiation treatment. Furthermore, we demonstrate that our photochemical approach for doping of graphene allows one to optically write doping structure with spatial control. Given the ease, effectiveness, and simplicity of the method, this photochemical doping mechanism offers a simple, reversible approach to control the steady-state electronic and optical properties of graphene.

Photochemical Doping of Protonic Transistors from a Cephalopod Protein

The field of bioelectronics has the potential to revolutionize both fundamental biology and personalized medicine. As such, much research effort has been devoted to the development of devices and materials that are intrinsically compatible with biological systems. Within this context, several recent studies have focused on protonic transistors from naturally occurring materials, such as squid-derived polysaccharides and proteins. Herein, we report the rational design, fabrication, and characterization of two- and three-terminal protonic devices, for which the active material consists of a protein-based proton conductor doped with a small molecular photoacid. We electrically interrogate these devices both in the absence and presence of illumination, demonstrating that an exogenous photophysical stimulus selectively enhances their electrical properties. Our findings hold significance for understanding and controlling proton transport not only in bioelectronic platforms but also across a wide range of voltage-regulated proton-conducting materials and device platforms.

Photochemical doping of graphene oxide thin film with nitrogen for photoconductivity enhancement

N-doped graphene oxide (NGO) thin film is synthesized by irradiation of graphene oxide (GO) thin film in NH3 atmosphere. NGO thin film obtained by irradiation of GO thin film for 60min has a high N-doping level of 12.69at.%, and the amino-like N dominates the doping with the level of 7.90at.%. The photoconductivity properties of NGO thin film under white-light illumination have been systematically examined. The results show that compared to reduced graphene oxide (rGO) thin film, NGO thin film exhibits significant photoconductivity enhancement with a high ratio of 2000%, and shows a faster photoresponse. The current responsivity and external quantum efficiency values for the NGO film reach ∼31mAW−1 and ∼87% at 2V, respectively. It may attribute to the high doping level of amino-like N on the basal plane and the pyridine-like N at the vacancy-site of graphene sheets, which can offer substantial photocarriers and the effective transfer to the electrodes.

Nitrogen-doped graphene films from simple photochemical doping for n-type field-effect transistors

Highly nitrogen-doped GO (NGO) and n-type graphene field effect transistor (FET) have been achieved by simple irradiation of graphene oxide (GO) thin films in NH3 atmosphere. The electrical properties of the NGO film were performed on electric field effect measurements, and it displays an n-type FET behavior with a charge neutral point (Dirac point) located at around −8 V. It is suggested that the amino-like nitrogen (N-A) mainly contributes to the n-type behavior. Furthermore, compared to the GO film irradiated in Ar atmosphere, the NGO film is much more capable to improve the electrical conductivity. It may attribute to nitrogen doping and oxygen reduction, both of which can effectively enhance the electrical conductivity.

Photochemical doping of graphene oxide thin films with nitrogen for electrical conductivity improvement

By using simple irradiation of graphene oxide (GO) thin films in NH3 atmosphere, nitrogen-doped GO (NGO) thin films had been successfully synthesized. It is notable that the reduction and nitrogen doping of graphene are simultaneously achieved by irradiation, and the nitrogen content of NGO could reach as high as 12.34 at.% within only 5 min. With further prolonging the irradiation time to 120 min, it could reach a very high value of 17.21 at.%. The electrical properties of NGO thin films were investigated. The results showed that, compared to irradiation of GO films in Ar atmosphere, irradiation in NH3 atmosphere is much more effective to improve the electrical conductivity. It may attribute to the nitrogen doping and oxygen reduction, both of which can effectively enhance the electrical conductivity.

Photochemical Doping and Tuning of the Work Function and Dirac Point in Graphene Using Photoacid and Photobase Generators

This work demonstrates that photochemical doping of CVD‐grown graphene can be easily achieved using photoacid (PAG) and photobase (PBG) generators such as triphenylsulfonium perfluoro‐1‐butanesufonate (TPS‐Nf) and 2‐nitrobenzyl N‐cyclohexylcarbamate (NBC). The TPS‐Nf ionic onium salt photoacid generator does not noticeably dope or alter the electrical properties of graphene when coated onto the graphene surface, but is very effective at inducing p‐doping of graphene upon exposure of the PAG‐coated graphene sample. Likewise, the neutral NBC photobase generator does not significantly affect the electrical properties of graphene when coated, but upon exposure to ultraviolet light produces a free amine, which induces n‐doping of the graphene. Electrical measurements show that the doping concentration can be modulated by controlling the deep ultraviolet (DUV) light exposure dose delivered to the sample. The interaction between both dopants and graphene is also investigated. The photochemical doping process is able to tune the work function of the single‐layer graphene samples used in this work from 3.4 eV to 5.3 eV. Finally, a p–n junction is fabricated and analyzed, showing that it is possible to control the position of the two current minima (two Dirac points) in the ambipolar p–n junction.

Photochemical doping of graphene oxide with nitrogen for photoluminescence enhancement

Nitrogen-doped graphene oxide (NGO) was synthesized by irradiation of graphene oxide (GO) in NH3 atmosphere. NGO obtained by irradiation of GO for 10 min has high N content of 13.62 at. %. The photoluminescence (PL) properties of NGO were investigated. The results showed that compared with GO, NGO exhibits significant PL enhancement with a high enhancement ratio of approximately 1501.57%. It may attribute to the high content of amino-like N, which can effectively enhance PL of GO because of the amino conjugation effect.

Photochemical Doping of an Adaptive Mix-Conducting p−n Junction

Existing photovoltaic junctions typically contain only electronic charge carriers, and they rely on the built-in electronic asymmetry between constituent materials to separate photogenerated electrons and holes. In this Letter, we report the observation of a photovoltaic response and photochemically induced near-infrared absorption from a mixed ionic−electronic junction constructed from two intrinsic polyacetylene ionomers with oppositely charged mobile counterions. We propose that the asymmetry in ionic charge carriers across the junction leads to the separation of photogenerated carriers and the in situ photochemical doping of both ionomers under illumination. Unlike purely electronic junctions, the doping level of mixed ionic−electronic junctions can change adaptively with light intensity due to the interaction between ionic and electronic carriers. The unique working mechanism of mix-conducting junctions could be used for designing solar cells and adaptive photoelectronic devices.