Pentacene Molecules Research Articles

Room-temperature cavity quantum electrodynamics with strongly coupled Dicke states

The strong coupling regime is essential for efficient transfer of excitations between states in different quantum systems on timescales shorter than their lifetimes. The coupling of single spins to microwave photons is very weak but can be enhanced by increasing the local density of states by reducing the magnetic mode volume of the cavity. In practice, it is difficult to achieve both small cavity mode volume and low cavity decay rate, so superconducting metals are often employed at cryogenic temperatures. For an ensembles of N spins, the spin–photon coupling can be enhanced by sqrt N through collective spin excitations known as Dicke states. For sufficiently large N the collective spin–photon coupling can exceed both the spin decoherence and cavity decay rates, making the strong-coupling regime accessible. Here we demonstrate strong coupling and cavity quantum electrodynamics in a solid-state system at room-temperature. We generate an inverted spin-ensemble with N ~ 1015 by photo-exciting pentacene molecules into spin-triplet states with spin dephasing time T_2^*sim 3 μs. When coupled to a 1.45 GHz TE01δ mode supported by a high Purcell factor strontium titanate dielectric cavity (V_{mathrm{m}}sim 0.25 cm3, Q ~ 8,500), we observe Rabi oscillations in the microwave emission from collective Dicke states and a 1.8 MHz normal-mode splitting of the resultant collective spin–photon polariton. We also observe a cavity protection effect at the onset of the strong-coupling regime which decreases the polariton decay rate as the collective coupling increases.

Lattice-Directed Construction of Metal–Organic Molecular Wires of Pentacene on the Au(110) Surface

The construction of metal–organic molecular wires is important for the design of specific functional devices but has been a great challenge for experimental technology. Here we report the formation of one-dimensional metal–organic structures by direct deposition of pentacene molecules on the Au(110) surface with subsequent thermal annealing. These metal–organic molecular wires were systematically explored by scanning tunneling microscopy (STM) and density functional theory calculations. At submonolayer coverage, during annealing at ∼470 K, the adsorbed molecules induce both Au(110)-(1 × 3) surface reconstruction, where two atomic rows are missing every three rows on the Au(110) surface, with the end-to-end pentacene configuration and Au(110)-(1 × 6) surface reconstruction, where five rows are missing every six rows on the surface, with the side-by-side configuration. Further annealing at ∼520 K results in Au-adatom-coordinated metal–organic molecular wires with a new side-by-side configuration of pentacen...

Growth of pentacene on α−Al2O3 (0001) studied by in situ optical spectroscopy

The growth of pentacene thin films on a sapphire $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$(0001) surface was investigated in situ using differential reflectance spectroscopy (DRS). Two different film structures are observed depending on the substrate temperature. If pentacene is deposited at room temperature, a wetting layer consisting of flat-lying molecules is formed after which upright-standing molecular layers with a herringbone structure start to grow. At low substrate temperature of 100 K, the long molecular axis of the pentacene molecules remains parallel to the surface plane throughout the entire growth regime up to rather large thicknesses. Heating thin films deposited at 100 K to room temperature causes the pentacene molecules beyond the wetting layer to stand up and assemble into a herringbone structure. Another interesting observation is the dewetting of the first flat-lying monolayer upon exposure to air, leading to the condensation of islands consisting of upright-standing molecules. Our results emphasize the interplay between growth kinetics and thermodynamics and its influence on the molecular orientation in organic thin films.

Angular-Dependent EDMR Linewidth for Spin-Dependent Space-Charge-Limited Conduction in a Polycrystalline Pentacene

Spin-dependent space charge limited carrier conduction in a Schottky barrier diode using polycrystalline \emph{p}-type $\pi$-conjugated molecular pentacene is explored using multiple-frequency electrically detected magnetic resonance (EDMR) spectroscopy with a variable-angle configuration. The measured EDMR spectra are decomposed into two components derived respectively from mobile and trapped positive polarons. The linewidth of the EDMR signal for the trapped polarons increases with increasing resonance magnetic field for an in-plane configuration where the normal vector of the device substrate is perpendicular to the resonance magnetic field, while it is independent of the field for an out-of-plane configuration. This difference is consistent with the pentacene arrangement on the device substrate, where pentacene molecules exhibit a uniaxial orientation on the out-of-substrate plane. By contrast, the mobile polarons do not show anisotropic behavior with respect to the resonance magnetic field, indicating that the anisotropic effect is averaged out owing to carrier motion. These results suggest that the orientational arrangements of polycrystalline pentacene molecules in a nano thin film play a crucial role in spin-dependent electrical conduction.

Frontier molecular orbitals of a single molecule adsorbed on thin insulating films supported by a metal substrate: electron and hole attachment energies

We present calculations of vertical electron and hole attachment energies to the frontier orbitals of a pentacene molecule absorbed on multi-layer sodium chloride films supported by a copper substrate using a simplified density functional theory (DFT) method. The adsorbate and the film are treated fully within DFT, whereas the metal is treated implicitly by a perfect conductor model. We find that the computed energy gap between the highest and lowest unoccupied molecular orbitals—HOMO and LUMO -from the vertical attachment energies increases with the thickness of the insulating film, in agreement with experiments. This increase of the gap can be rationalised in a simple dielectric model with parameters determined from DFT calculations and is found to be dominated by the image interaction with the metal. We find, however, that this simplified model overestimates the downward shift of the energy gap in the limit of an infinitely thick film.

Oxidized pentacene micro-rods obtained by thermal annealing of pentacene thin films in air

Prolonged annealing of pentacene thin films in air leads to the formation of nano- and micro-scale rod-shaped structures at temperatures equal to or higher than 130 °C. Scanning electron microscopy measurements indicated their crystalline structure, while UV–vis absorption spectra revealed presence of different species of oxidized pentacene, including 6,13-pentacenequinone. The mechanism of growth of microcrystals from oxidized pentacene molecules is discussed. Raman and UV–vis absorption spectra dependences on film thickness (in 30–300 nm range) and on thermal annealing conditions (in air and nitrogen at ambient pressure at 100 and 150 °C) were also studied. These spectra are not largely affected by annealing if it is performed in nitrogen at any of studied temperatures and annealing times (few hours to few days). However, if annealing is performed in air, at temperatures 130 °C and higher, changes in spectral features are significant due to film oxidation.

Pentacene crystal transition during the growth on SiO2 studied by in situ optical spectroscopy

We discuss the processes involved in the formation of a few monolayer thick pentacene films on the SiO2 surface. The evolution of the optical properties has been monitored in situ by optical spectroscopy and the interface morphology was investigated ex situ by atomic force microscopy. The sub monolayer shows the identical characteristic optical response with the intensity proportional to the coverage. From the second monolayer growth on, the optical properties are significantly changed. This experimental evidence indicates the formation of the first monolayer layer comprising the so-called “orthorhombic phase” crystallites with near-vertically oriented pentacene molecules, and the multilayer above comprises the so-called “thin-film phase” crystallites. Most importantly, the combination of optical spectroscopy and atomic force microscopy (AFM) investigations reveals that the first monolayer crystallize into the orthorhombic phase and the structure is persevered even after being buried by the multilayer. These observations suggest that the thickness-driven phase transformation from the orthorhombic phase to the thin-film phase starts from the second monolayer on rather than the first monolayer. This result provides a new insight into the structural evolutions and growth of pentacene thin films on SiO2.

Adsorption and electronic properties of pentacene on thin dielectric decoupling layers.

With the increasing use of thin dielectric decoupling layers to study the electronic properties of organic molecules on metal surfaces, comparative studies are needed in order to generalize findings and formulate practical rules. In this paper we study the adsorption and electronic properties of pentacene deposited onto h-BN/Rh(111) and compare them with those of pentacene deposited onto KCl on various metal surfaces. When deposited onto KCl, the HOMO and LUMO energies of the pentacene molecules scale with the work functions of the combined KCl/metal surface. The magnitude of the variation between the respective KCl/metal systems indicates the degree of interaction of the frontier orbitals with the underlying metal. The results confirm that the so-called IDIS model developed by Willenbockel et al. applies not only to molecular layers on bare metal surfaces, but also to individual molecules on thin electronically decoupling layers. Depositing pentacene onto h-BN/Rh(111) results in significantly different adsorption characteristics, due to the topographic corrugation of the surface as well as the lateral electric fields it presents. These properties are reflected in the divergence from the aforementioned trend for the orbital energies of pentacene deposited onto h-BN/Rh(111), as well as in the different adsorption geometry. Thus, the highly desirable capacity of h-BN to trap molecules comes at the price of enhanced metal–molecule interaction, which decreases the HOMO–LUMO gap of the molecules. In spite of the enhanced interaction, the molecular orbitals are evident in scanning tunnelling spectroscopy (STS) and their shapes can be resolved by spectroscopic mapping.

Single-Molecule Level Insight into Nanoscale Environment-Dependent Photophysics in Blends

Organic semiconductors have attracted considerable attention due to their applications in low-cost (opto)electronic devices. Many successful organic materials utilize blends of several types of molecules that contribute different functions (e.g., serving as donors and acceptors in solar cells). In blends, the local environment, which is inherently heterogeneous, strongly influences the (opto)electronic performance and photostability. We use functionalized fluorinated pentacene (F8 TCHS-Pn) molecules as single-molecule probes of the nanoscale environment in blends containing donor and acceptor molecules incorporated into a polymer (PMMA) matrix. Single F8 TCHS-Pn donor (D) molecules were imaged in PMMA in the presence of functionalized indenofluorene (TIPS-IF) or PCBM acceptor (A) molecules using wide-field fluorescence microscopy at various concentrations. Long-lived dark states attributed to a reversible formation of an endoperoxide (TCHS-EPO) were observed, and the EPO formation and reversal processes, ...

A comparison study of electrode material effects on the molecular single electron transistor

In this paper, the studies of Molecular Single electron Transistor (MSET) is presented. We have used the Extended Huckel Theory (EHT) coupled with non-equilibrium green's function (NEGF) formalism which is implemented in ATOMISTIC TOOLKIT package. The transport properties with different electrode materials have been studied for the first time using pentacene molecule. The presence of Coulomb blockade on I–V characteristics confirmed that our device operates as Single Electron Transistor. The negative differential resistance (NDR) effect and the coulomb Staircase state has been outlined. The transmission Spectra and density of states (DOS) dependence of the electrode has also been explored to analyze the I-V curves. The NDR behavior and size device make our MSET as a good candidate for use in gate logic circuit with low consumption.

Charge-Transfer States in Organic Solar Cells: Understanding the Impact of Polarization, Delocalization, and Disorder.

We investigate the impact of electronic polarization, charge delocalization, and energetic disorder on the charge-transfer (CT) states formed at a planar C60/pentacene interface. The ability to examine large complexes containing up to seven pentacene molecules and three C60 molecules allows us to take explicitly into account the electronic polarization effects. These complexes are extracted from a bilayer architecture modeled by molecular dynamics simulations and evaluated by means of electronic-structure calculations based on long-range-separated functionals (ωB97XD and BNL) with optimized range-separation parameters. The energies of the lowest charge-transfer states derived for the large complexes are in very good agreement with the experimentally reported values. The average singlet-triplet energy splittings of the lowest CT states are calculated not to exceed 10 meV. The rates of geminate recombination as well as of dissociation of the triplet excitons are also evaluated. In line with experiment, our results indicate that the pentacene triplet excitons generated through singlet fission can dissociate into separated charges on a picosecond time scale, despite the fact that their energy in C60/pentacene heterojunctions is slightly lower than the energies of the lowest CT triplet states.

Self-alignment of 6,13-bis(triisopropylsilylethynyl)pentacene molecules through magnetic flux-affected nanoparticle motion in solution-processed transistors

We propose a viable method of inducing the self-alignment of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) molecules using magnetic nanoparticles under a controlled magnetic field. An essential feature of the self-alignment of TIPS-pentacene containing magnetic nanoparticles is systematically studied through meticulous understanding of gradient solvent evaporation in a magnetic nanoparticle-dispersed droplet guided by a static magnetic field. It is found that such TIPS-pentacene molecules are spontaneously aligned by magnetic flux-affected nanoparticle motion with the generation of gradual anisotropic solvent evaporation. In particular, by controlling the effective magnetic force applied to a magnetic nanoparticle-containing TIPS-pentacene solution, TIPS-pentacene droplet flow can be induced, causing greater alignment. We fabricate TIPS-pentacene-based transistors to confirm the effect of this aligned state on device performance, using both optical and electrical methods.

Real time monitoring of ordering in pentacene films during growth by using in-situ infrared spectroscopy

By employing the in-situ Fourier transform infrared (FTIR) spectroscopy and curve-fitting, we studied the evolution in the ordering of vacuum-deposited pentacene films. In the FTIR spectra, the thin film and bulk phases appeared at 903.2 cm−1 and 907.2 cm−1, respectively. The misoriented phase was also observed at 904.4 cm−1. The position and the intensity of each phase varied with the film thickness as well as the growth temperature. The results indicated that most of pentacene molecules formed initially the thin film phase until the bulk and misoriented phases began to form at the increased thickness. Our study suggests that FTIR spectroscopy is an effective tool to monitor in real time the ordering of organic films for quality control.

Crystallographic polarity effect of ZnO on thin film growth of pentacene

The spontaneous polarization effect of ZnO on the thin film growth of pentacene, which is a typical π conjunction organic semiconductor, was investigated. Pentacene thin films were grown on polar ZnO surfaces by ultraslow organic film physical vapor deposition to obtain layer-by-layer growth. X-ray diffraction measurements revealed that pentacene molecules stand upright on polar ZnO surfaces, and that the films consist of two polymorphs, namely, the thin-film and bulk phases. The thin-film phases of pentacene were observed regardless of the polarity of the ZnO substrate at a thickness of less than six molecular layers. However, pentacene on a Zn-polar ZnO substrate gradually changed to the bulk phase unlike that on an O-polar ZnO substrate. Kelvin probe force microscopy measurements revealed that the surface potential of pentacene becomes more positive with increasing pentacene thickness at less than two molecular layers. The variation in the potential of pentacene on the Zn-polar ZnO substrate was larger than that of pentacene on the O-polar ZnO substrate. These findings indicate that the polarity of the semiconductor can control the growth and electronic state of the inorganic/organic semiconductor interface.

Electrode configurations for pentacene-deposited organic thin film transistors

ABSTRACTWhen a pentacene OTFT is fabricated with a bottom gate structure, the top-contact configuration of gold electrodes is preferred because it results in better electrical performance than the bottom-contact configuration. Pentacene molecules are deposited only on the SiO2 dielectric layer in the top-contact configuration, and thus all pentacene molecules form a vertical orientation that induces efficient charge transport between molecules. However, in an OTFT device with the bottom-contact configuration, pentacene molecules adopt a random tilted orientation at the boundary of the gold electrode, resulting in poor charge transport between molecules.

Charge-transfer states and optical transitions at the pentacene-TiO2 interface

Pentacene molecules have recently been observed to form a well-ordered monolayer on the (110) surface of rutile TiO2, with the molecules adsorbed lying flat, head to tail. With the geometry favorable for direct optical excitation and given its ordered character, this interface seems to provide an intriguing model to study charge-transfer excitations where the optically excited electrons and holes reside on different sides of the organic–inorganic interface. In this work, we theoretically investigate the structural and electronic properties of this system by means of ab initio calculations and compute its excitonic absorption spectrum. Molecular states appear in the band gap of the clean TiO2 surface, which enables charge-transfer excitations directly from the molecular HOMO to the TiO2 conduction band. The calculated optical spectrum shows a strong polarization dependence and displays excitonic resonances corresponding to the charge-transfer states, which could stimulate new experimental work on the optical response of this interface.

The effect of interface-induced structural properties of the pentacene accumulation layer on the threshold voltage: Pentacene monolayer transistors

The effect of pentacene/gate dielectric interface-induced structural properties of a pentacene monolayer (ML) close to the SiO2 gate dielectric on the threshold voltage in the field effect transistor (FET) and van der Pauw was addressed using atomic force microscopy and near edge X-ray absorption fine structure spectroscopy (NEXAFS). Our study reveals that large negative threshold voltage found in FET and van der Pauw devices is not closely correlated to the molecular orientation and the grain size of pentacene molecules in direct contact with the SiO2 gate dielectric. In concluding our finding, NEXAFS results in the ultrathin pentacene film within the carrier accumulation thickness regime, characterized by Debye length, were correlated to the magnitude of the threshold voltage from field effect devices. Our work motivates finding and visualizing interface-induced structural properties that are directly correlated to the magnitude of the threshold voltage.

Charge Transfer and Interface Engineering of the Pentacene and MoS2 Monolayer Complex

Molecular doping of monolayer MoS2 provides a great opportunity to modulate its electronic properties for the potential applications in high performance devices. Density functional theory computations are performed to investigate the charge transfer and electrostatic potential modulation upon the adsorption of pentacene molecule on the surface of MoS2 monolayer (ML). Theoretical calculations indicate that interfacial charge transfer is negligible between pentacene and 2H‐MoS2 ML while significant in the pentacene/1T‐MoS2 ML complex, which is attributed to the match of energy levels near the Fermi level in the latter case. Moreover, molecular doping of pentacene induces substantial structure changes of the substrate resulting in large adsorption energy, which helps stabilize the 1T‐MoS2 ML. Depending on different substrate phases and doping configurations, the interfacial dipole barrier and related work function of MoS2 ML may be modulated in a wide range of the order of about 1 eV. The findings therefore shed light on the possibility of developing the desired organic/inorganic complex for electrical and optoelectronic devices by molecular doping via charge transfer modulation and interface engineering.

Adiabatic electron affinity of pentacene and perfluoropentacene molecules studied by anion photoelectron spectroscopy: Molecular insights into electronic properties.

Pentacene (C22H14, PEN) and perfluoropentacene (C22F14, PFP) are considered promising building blocks of organic semiconductors. Using gas-phase anion photoelectron spectroscopy, the adiabatic electron affinity of PEN and PFP molecules is determined to be 1.43 ± 0.03 and 2.74 ± 0.03 eV, respectively, and the S0-T1 transition energies of PEN and PFP are evaluated to be 0.96 ± 0.06 and 0.72 ± 0.05 eV, respectively. Photoelectron spectra indicate that the vibronic coupling in PFP is stronger than that in PEN. Quantum chemistry calculations demonstrate that the strong vibronic coupling originates from significant structural displacement upon electron injection to PFP.

The direct exchange mechanism of induced spin polarization of low-dimensional π-conjugated carbon- and h-BN fragments at LSMO(001) MnO-terminated interfaces

Induced spin polarization of π-conjugated carbon and h-BN low dimensional fragments at the interfaces formed by deposition of pentacene molecule and narrow zigzag graphene and h-BN nanoribbons on MnO2-terminated LSMO(001) thin film was studied using GGA PBE+U PAW D3-corrected approach. Induced spin polarization of π-conjugated low-dimensional fragments is caused by direct exchange with Mn ions of LSMO(001) MnO-derived surface. Due to direct exchange, the pentacene molecule changes its diamagnetic narrow-band gap semiconducting nature to the ferromagnetic semiconducting state with 0.15eV energy shift between spin-up and spin-down valence bands and total magnetic moment of 0.11μB. Direct exchange converts graphene nanoribbon to 100% spin-polarized half-metal with large amplitude of spin-up electronic density at the Fermi level. The direct exchange narrows the h-BN nanoribbon band gap from 4.04 to 1.72eV in spin-up channel and converts the h-BN ribbon semiconducting diamagnetic nature to a semiconducting magnetic one. The electronic structure calculations demonstrate a possibility to control the spin properties of low-dimensional π-conjugated carbon and h-BN fragments by direct exchange with MnO-derived LSMO(001) surface for spin-related applications.