Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Dual-wavelength control of charge accumulation in rubrene microcrystals with anisotropic conductivity

Moha Naeimi, Ingo Barke, Sylvia Speller

Abstract

Previously, a novel type of rubrene microcrystals was reported, forming two distinct sectors -- diamond- and triangular-shaped -- that exhibit pronounced contrasts in photoluminescence (PL) spectra and exciton dynamics. In the present work, their internal electronic structure is investigated using time-of-flight photoemission electron spectroscopy (TOF-PES), revealing that the two sector's different charging characteristics arising from anisotropic conductivities. Upon photoemission via a one-photon photoemission (1PPE) process excited by 6.2 eV (200 nm) photons, the diamond-shaped sectors accumulate significant charge, whereas the triangular sectors remain essentially uncharged. The charge accumulation in the diamond sectors can be neutralized by additional sub-threshold illumination, which generates charge carriers through internal photoeffect. The dynamics and energetics of the observed band shifting is described quantitatively by a model combining surface capacitance and drift-diffusion. These crystalline systems enable the creation of built-in charge landscapes that can be manipulated both spatially and temporally.

Dual-wavelength control of charge accumulation in rubrene microcrystals with anisotropic conductivity

Abstract

Previously, a novel type of rubrene microcrystals was reported, forming two distinct sectors -- diamond- and triangular-shaped -- that exhibit pronounced contrasts in photoluminescence (PL) spectra and exciton dynamics. In the present work, their internal electronic structure is investigated using time-of-flight photoemission electron spectroscopy (TOF-PES), revealing that the two sector's different charging characteristics arising from anisotropic conductivities. Upon photoemission via a one-photon photoemission (1PPE) process excited by 6.2 eV (200 nm) photons, the diamond-shaped sectors accumulate significant charge, whereas the triangular sectors remain essentially uncharged. The charge accumulation in the diamond sectors can be neutralized by additional sub-threshold illumination, which generates charge carriers through internal photoeffect. The dynamics and energetics of the observed band shifting is described quantitatively by a model combining surface capacitance and drift-diffusion. These crystalline systems enable the creation of built-in charge landscapes that can be manipulated both spatially and temporally.
Paper Structure (8 sections, 4 equations, 5 figures)

This paper contains 8 sections, 4 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Experimental methods
  3. Results and discussion
  4. RC charging model
  5. Drift diffusion correction
  6. Surface potential investigations
  7. Conclusion
  8. Acknowledgement

Figures (5)

  • Figure 1: (a) 1PPE electron spectra of a rubrene crystal resolved for different sectors 6.2 eV illumination. (b) 1PPE electron spectra of the same crystal upon two colour illumination (6.2 eV and 3.1 eV). The insets are PEEM maps which colour-code the spatially-resolved centre of spectral weight. The charge patterns resemble the diamond- and triangular-shaped sectors. (scale bar: 10 $\mu$m) (c and d) Schematic of the orthorhombic ac-plane in diamond- and triangular-shaped sectors, respectively. The blue arrows and circles, represent the excitation light and polarization plane, applied with the angle of $23^\circ$ grazing. Note that the spectra for the diamond sector are shifted towards higher "apparent" binding energies.
  • Figure 2: It takes some time for the crystal to fully charge. (a) Electron spectra of the crystal upon illumination (6.16 eV) for different times after illumination is started. (b) Shift of spectra (charge) versus time after illumination has started for two different illumination powers.
  • Figure 3: Different crystal domains are charge differently and the more excitation intensity is, the more charge is stored. (a) Overall electron spectra of the crystal for different illumination powers. The dashed lines indicate the sector and substrate specific spectral bands. (b) Spectral shift versus the UV illumination average power, resolved for different sectors. (c) Schematic of single colour experiment. left panel: Red and green rectangles represent the diamond- and triangular-shaped sectors. The blue and green circles represent the photoelectrons and electron supplement from the substrate. The positive charge accumulation near surface is indicated with + sign. right panel: Schematic of RC model with $\text{R}_0$ and $\beta \text{I}_{UV}$ representing the dark resistance of rubrene crystal and resistance induced by UV illumination, respectively.
  • Figure 4: The Vis light can suppress the charge induced by the UV light. (a) Electron spectra of rubrene single crystals illuminated simultaneously by UV (1.4 nW) and different Vis powers. (b and c) Spectral shift versus Vis illumination power, resolved for different sectors. (d) Schematics of double-colour experiment. left panel: Red and green rectangles represent the diamond- and triangular-shaped sectors. The blue and green circles represent the photoelectrons and electron supplement from the substrate. The black and white circles indicate the excitons generated via internal photoelectric upon Vis light. right panel: Schematics of corresponding RC model with $R_0$ representing the dark resistance, $\beta I_{UV}$ and $\gamma I_{vis}$ being the photo resistance due to UV and Vis illumination, respectively.
  • Figure 5: (a) KPFM map of a rubrene crystal showing the contrast of diamond- and triangular-shaped zones, indicating different intrinsic work function of different zones. (b) KPFM potential of a rubrene microcrystal versus grazing illumination intensity, resolved for different sectors.