Molecular Photodiode Research Articles

Molecular Van Der Waals Heterojunction Photodiodes Enabling Dipole-Induced Polarity Switching.

Solid-state devices capable of controlling light-responsive charge transport at the molecular scale are essential for developing molecular optoelectronic technology. Here, a solid-state molecular photodiode device constructed by forming van der Waals (vdW) heterojunctions between standard molecular self-assembled monolayers and two-dimensional semiconductors such as WSe2 is reported. In particular, two non-functionalized molecular species used herein (i.e., tridecafluoro-1-octanethiol and 1-octanethiol) enable bidirectional modulation of the interface band alignment with WSe2 , depending on their dipole orientations. This dipole-induced band modulation at the vdW heterointerface leads to the opposite change of both photoswitching polarity and rectifying characteristics. Furthermore, compared with other molecular or 2D photodiodes at a similar scale, these heterojunction devices exhibit significantly enhanced photo-responsive performances in terms of photocurrent magnitude, open-circuit potential, and switching speed. This study proposes a novel concept of the solid-state molecular optoelectronic device with controlled functions and enhanced performances.

Control of electroluminescence in a molecular photodiode by gate voltage

The role of the gate voltage in the regulation of electroluminescence (EL) of a molecular photodiode with asymmetric localization of electron density on the frontier highest occupied and lowest unoccupied molecular orbitals of the photochromic molecule is considered. It is shown that the gate voltage can have a significant effect on the formation of EL in devices where one of the orbital energy levels are outside the gap between the biased Fermi levels of the electrodes. The role of the gate voltage consists in shifting the position of the orbital energy levels until both frontier levels fall into the gap and thereby provide a resonant mechanism for the formation of EL. This leads to the inclusion of EL at a lower bias voltage than that which includes EL at zero gate voltage. In addition, the shift in energy levels caused by the gate voltage explains the mechanism for controlling the kinetics of bipolarity formation. The effect is carried out by turning on and off the resonant hopping’s of the electron between the conducting states of the electrodes and molecular orbitals localized at different distances from the electrode surfaces.

Microscopic and spectroscopic properties of Langmuir–Blodgett films composed of flavins and their aggregation structures

Isoalloxazine derivatives (flavins) are commonly found in natural systems that are involved in an electron transfer process, such as photosynthetic or metabolic systems, and are also frequently used as electron donors in organic-based electronic devices. As an example, molecular photodiodes composed of 7,8-dimethyl-10-dodecyl isoalloxazine (DDI) have been fabricated by the Langmuir–Blodgett (LB) technique, and such devices showed characteristic properties of photodiodes. The efficiency of molecular photodiodes is dependent on the assembled structure of the LB films, which is related to the morphology of the LB films. For that reason, Lim has investigated the morphology of LB films, and found that rod-shaped domains are formed when a DDI monolayer is transferred to a solid substrate above a specific surface pressure (Thin Solid Films, 531 (2013) 499). In that paper, rod-shaped domains were revealed to be collapsed triple layers, i.e., double layers collapsed on the monolayer; however, the detailed aggregation structure of the constituent molecules (DDI) has not been studied. Herein, we investigate the microscopic and spectroscopic properties of LB films composed of DDI. We apply the extended dipole model to explain spectral changes in the absorption spectra and propose an aggregation structure for DDI in the LB films.

Transient currents in a molecular photo-diode

Light-induced charge transmission through a molecular junction (molecular diode) is studied in the framework of a HOMO–LUMO model and in using a kinetic description. Expressions are presented for the sequential (hopping) and direct (tunneling) transient current components together with kinetic equations governing the time-dependent populations of the neutral and charged molecular states which participate in the current formation. Resonant and off-resonant charge transmission processes are analyzed in detail. It is demonstrated that the transient currents are associated with a molecular charging process which is initiated by photo excitation of the molecule. If the coupling of the molecule to the electrodes is strongly asymmetric the transient currents can significantly exceed the steady state current.

Molecular Photodiode and Two-channel Optoelectronic Demultiplexer based on the [60]Fullerene-porphyrin Tetrad

Photoelectrodes containing Langmuir–Blodget layers of [60]fullerene-porphyrin tetrad behave like photodiodes. Upon excitation within the whole absorption spectrum of the molecule they generate photocurrent, the direction of which depends on the conducting substrate potential. At negative polarization high intensity cathodic photocurrent are observed, while at positive polarization much weaker anodic photocurrents are observed. The forward-bias to reverse-bias current ratio amounts 5:1. Therefore the [60]fullerene-porphyrin tetrad is closely related to semiconductors showing photoelectrochemical photocurrent switching effect and is a promising material for molecular optoelectronics. It can be used as a simple molecular photodiode. Assignment of logic values to polarization of the photoelectrode and to light and photocurrent pulses results in a very efficient two-channel optoelectronic demultiplexer.

Excited state relaxation properties of donor–acceptor-bithiophene and related compounds

Thiophene containing biaryls of ionic and nonionic character are investigated with respect to their photophysical properties and their ground and excited state hypersurfaces. The ionic dimethylamino-thiophene-pyridinium derivative showed a strong nonradiative deactivation channel assigned to intramolecular twisting and charge shift by analogy with a related biphenyl-type compound and based on the quantum chemical results. Implications for the induction of molecular photodiode behaviour and the possible construction of molecular photoswitches are discussed.

A molecular photodiode system that can switch photocurrent direction.

We prepared a molecular photodiode system in which the photocurrent direction can be switched by choosing the wavelength of an irradiating light. The molecular system is composed of two types of helical peptides that carry different chromophores and have different directions of dipole moments when they are immobilized on gold. The mixed, self-assembled monolayer generated an anodic photocurrent when one of the two chromophores was photoexcited, whereas the photocurrent switched to being cathodic when the other chromophore was photoexcited. The opposite current response arises from the dipole moment of each helical peptide, which accelerates electron transfer in the same direction.

Molecular-scale biophotodiode consisting of a green fluorescent protein/cytochrome c self-assembled heterolayer

A molecular photodiode that consists of a protein heterolayer is investigated at the molecular scale for construction of a bioelectronic device. Cytochrome c and green fluorescent protein (GFP) were used as an electron acceptor and a sensitizer in the molecular layer by mimicking photosynthesis. A self-assembled monolayer of thiol-modified cytochrome c was formed on Au-coated glass, and then GFP was adsorbed onto the cytochrome c surface by electrostatic attraction. Photoinduced current was generated and the photoswitching property was observed by repeated step illumination. The rectifying property by scanning tunneling spectroscopy based current–voltage characteristics was achieved in the protein heterolayer. Thus, the proposed heterolayer functioned as a biomolecular photodiode with photocurrent generation and the rectifying property.

Rectified photocurrent in a protein based molecular photo-diode consisting of a cytochrome b562-green fluorescent protein chimera self-assembled monolayer

We fabricated a self-assembled monolayer (SAM) of a chimeric protein created as a novel model protein for an artificial light-harvesting complex (LHC) composed of two proteins, cytochrome b 562 (cyt b 562) mutated for SAM fabrication (cyt b 562, N22C, G82C) and enhanced green fluorescent protein (EGFP). The SAM formation of chimeric protein on a single-crystalline Au(1 1 1) substrate was confirmed by atomic force microscopy (AFM) measurement. The rectified photocurrent of the chimeric protein SAM on a gold substrate was detected by light-illumination scanning tunneling microscopy (LI-STM) co-operated with a lock-in technique. The photocurrent generation of the chimeric protein SAM was wavelength-specific to the light-illumination (488 nm), which indicated that the EGFP part of the chimera plays a role as a sensitizer in the photo-induced electron transfer process.

Investigation of charge transport in the molecular photodiode consisting of ferrocene/flavin/viologen/TCNQ molecular heterojunction

Abstract A molecular photodiode with the hetero-Langmuir–Blodgett (LB) film consisting of an electron donor (D), sensitizer (S), relay (R), and acceptor (A) was fabricated. Ferrocene octadecyl amine (Ferrocene), 7,8-dimethyl-10-dodecyl isoalloxazine (flavin), N-Allyl-N′-[3-propylamido-N″,N″-di(n-octadecyl)]-4,4′-bipyridium dibromide (viologen) and N-docosilquinolinium tetracyanodimethan (TCNQ) were used as the D, S, R, and A units, respectively. By aligning the hetero-LB film of the D/S/R/A units on indium tin oxide (ITO) coated glass with an aluminum thin film, a molecular photodiode with a Metal/Insulator/Metal (MIM) structure was constructed. The decay of the transient photocurrent of MIM device was measured using a 355 nm Nd:YAG pulse laser as the light source and the charge transfer rate was obtained. The charge transfer rate was faster in the D/S/R/A heterojunction than in S/R and S/R/A heterojunction devices due to less recombination from the A to S and less back electron transport due to the existence of D molecules. Various effects such as the LB film thickness, interface structure, and Schottky barrier on the charge transfer rate were investigated. The results showed that the charge transport efficiency of the molecular photodiode became higher when the LB film thickness increased and the head/head interfaced structure of the S/R hetero LB films was adopted. Furthermore, improvement of the molecular device efficiency occurred when spacer molecules were adopted between the electrode and the LB film.

Photoinduced electron transfer in GFP/viologen/TCNQ structured hetero-LB film

The photoinduced electron transfer rate in hetero-Langmuir–Blodgett (LB) films, composed of electron sensitizing protein and electron accepting organics, was investigated. The hetero-LB film was fabricated by sequentially depositing green fluorescence protein (GFP), viologen, and TCNQ, which functioned as an electron sensitizer, a relay, and an acceptor, respectively. To verify the hetero-LB film formation, surface morphologies of the GFP film and viologen/TCNQ LB film were observed by atomic force microscopy (AFM). The fast charge separation of the molecular interface was investigated by analyzing the time-resolved fluorescence decays of GFP homo-, GFP/viologen hetero- and GFP/viologen/TCNQ hetero-LB film using a time-correlated single photon counting (TCSPC) method. In GFP/viologen hetero-films, the fluorescence decay of excited GFP was faster than that in the GFP homo-film. Based on the decay of fluorescence the charge separation rates of excited GFP were calculated. The fluorescence lifetime and charge separation rate of the GFP/viologen/TCNQ hetero-LB film increased slightly compared to that of the GFP/viologen hetero-LB film as a result of the addition of the TCNQ layer, electron acceptor. On the basis of its molecular interface charge separation behavior, the proposed molecular array can be used as a component of the molecular photodiode.

Biomolecular photodiode consisting of cytochrome c-adsorbed hetero-Langmuir–Blodgett films

Photoinduced electron transfer in biomolecular photodiode consisting of protein-adsorbed hetero-Langmuir–Blodgett (LB) films was investigated. Four kinds of functional molecules, cytochrome c, viologen, flavin, and ferrocene, were used as a second electron acceptor, a first electron acceptor, an electron sensitizer, and an electron donor, respectively. The hetero-LB film was fabricated by subsequently depositing ferrocene, flavin, and viologen onto the pretreated ITO glass. Cytochrome c-adsorbed hetero-LB films were prepared by soaking the hetero-LB films into the phosphate buffer solution containing cytochrome c. To verify the optimal adsorption conditions of cytochrome c molecules onto the viologen LB layers, the UV-absorption spectrum and atomic force microscopy observations of LB films were performed. Finally, the metal/insulator/metal structured molecular device was constructed by depositing aluminum onto the surface of the cytochrome c-adsorbed hetero-LB films. For photoelectric response properties, the current–voltage characteristic and photoswitching effect of the proposed molecular photodiode were investigated. To verify the charge shift, transient photocurrent of the molecular photodiode was measured.

Fractal-Time Response Function of GFP/Viologen/TCNQ Structured Molecular Photodiode

Fractal-time response of the molecular photodiode consisting of hetero-molecular film was investigated. GFP, viologen, and TCNQ were used in the molecular photo diode as an electron sensitizer, first electron acceptor and second electron acceptor, respectively. Based on decay time profile of transient photocurrent by 400 nm laser pulse irradiation, the decay time constant value a of molecular photodiode was obtained. With the decay time constant value, the simulation of the fractal-time response to the pulse train inputs corresponding to different ASCII codes was performed.

Fractal-Time Response Function of GFP/Viologen/TCNQ Structured Molecular Photodiode

Fractal-time response of the molecular photodiode consisting of hetero-molecular film was investigated. GFP, viologen, and TCNQ were used in the molecular photo diode as an electron sensitizer, first electron acceptor and second electron acceptor, respectively. Based on decay time profile of transient photocurrent by 400 nm laser pulse irradiation, the decay time constant value a of molecular photodiode was obtained. With the decay time constant value, the simulation of the fractal-time response to the pulse train inputs corresponding to different ASCII codes was performed.

Rectified photocurrent of molecular photodiode consisting of cytochrome c/GFP hetero thin films

Photoinduced electron transfer in the molecular electronic device consisting of protein-adsorbed hetero Langmuir–Blodgett (LB) film was investigated. Three kinds of functional molecules, cytochrome c, viologen, and green fluorescent protein (GFP) were used as an electron acceptor, a mediator, and a sensitizer, respectively. The hetero-LB film was fabricated by subsequently depositing cytochrome c and viologen onto the pretreated ITO or quartz glass. GFP adsorbed hetero-LB films were prepared by soaking the hetero-LB films into the buffer solution containing GFP. The MIM (metal/insulator/metal) structured molecular device was constructed by depositing aluminum onto the surface of the GFP-adsorbed hetero LB films. Due to the excitation by irradiation with a 460 nm monochromic light source, the photoinduced unidirectional flow of electrons in the MIM device could be achieved and was detected as photocurrents. The photoswitching function was achieved and the rectifying characteristic was observed in the molecular device. Based on the measurement of transient photocurrent of molecular device, the unidirectional flow of electrons was verified.

Shift Register Memory Function of Molecular Photodiode Consisting of Flavin/Viologen/TCNQ Molecular Hetero-LB Films

A shift register memory of molecular photodiode, consisting of flavin/viologen/TCNQ molecular hetcro-LB film, at the molecular level is described. In the shift register memory device, the free electrons of sensitizer molecules have an excited or ground (“1” or “0”) state and the electrons are shifted between the functional molecules with photoinduced electron transfer reactions. Based on the transient photocurrent measurement, the shift register memory functions of molecular photodiode were observed with nanosecond time resolution.

Artificial molecular photodiode consisting of GFP/viologen hetero-thin film

By mimicking the biological photosynthesis system, molecular photodiode was developed artificially using molecular array consisting of protein/organic molecular hetero-thin films. Functional organic molecule (viologen) and biomolecules (green fluorescent protein, GFP) were chosen and used as an electron acceptor (A) and the electron sensitizer (S), respectively. Based on the redox potential of each molecules obtained by cyclic voltammetry measurement, molecular thin films were successfully fabricated by aligning the viologen (A) Langmuir–Blodgett film (LB) onto ITO glass and subsequently adsorbing GFP (S) molecules onto the viologen LB film surface. By depositing aluminum (Al) onto GFP/viologen hetero-thin films, a metal/insulator/metal (MIM) structured device was constructed. When the forward bias was applied to the fabricated device in accordance with the energy profile of the prepared thin films, photoswitching function could be obtained.

Fabrication of Molecular Scale Photodiode using Phase Separation Technique

A molecular-level photodiode was fabricated using a phase separation technique. The separated phase of a stacked layer of flavin (sensitizer (S))-ODTCS (octadecyl trichlorosilane) mixture and PFCA (nonadecafluoro-decanoic acid) was constructed and observed by AFM. Based on this structure, a submicron scale electron donor (D)/S/relay (R)/acceptor (A) molecular photodiode was fabricated on the silicon wafer substrate by the LB method. The morphology of the device was confirmed by AFM. The photodiode effect of the molecular device was observed using STM. The molecular scale photodiode and photoswitching effect could be verified.

KELVIN PROBE FORCE MICROSCOPY OF MOLECULAR SURFACES

▪ Abstract The electrostatic force microscope is one of many specialized tip sensors used in near-field microscopy. This type of microscope is realized by applying a voltage on a conducting AFM tip. It can be used to image samples that present a distribution of electrical properties on inhomogeneous materials as well as on nanostructures. This microscopy technique has been used to probe phase separation, chemical recognition, molecular orientation, and photo-induced charge separation in molecular photodiodes in Langmuir-Blodgett films.

Photocurrent Characteristics of Ferrocene/Flavin/Viologen/TCNQ Heterojunction

A molecular photodiode was fabricated with the hetero-Langmuir-Blodgett (LB) film consisting of an electron donor (D). sensitizer (S), relay (R) and acceptor (A). Ferrocene, flavin, viologen and TCNQ were used as D, S, R and A units, respectively. By aligning hetero-LB film of D/S/R/A units on ITO glass with an aluminum thin film, a molecular photodiode with the structure of Metal/Insulator/Metal (MIM) was constructed. Due to excitation by irradiation with a 460 nm monochromatic light source, the photoswitching function was observed. And action spectrum of MIM device was measured. To investigate the photoswitching time and charge transport rate, transient photocurrent of MIM device was measured. Result shows that efficient charge transport from D to A in the heterojunction was occurred due to the existence of R and less recombination.