C O M P U T A T I O N
Low-cost, fast and easy production of germanium
nanostructures and interfacial electron transfer
dynamics of BODIPY–germanium nanostructure
system
Sabriye Acikgoz1,* , Hasan Yungevis1 , Emin O¨ zu¨nal1 , and Ays¸egu¨l S¸ahin1 1
Department of Metallurgical and Materials Engineering, Karamanog˘lu Mehmetbey University, 70100 Karaman, Turkey
Received:8 June 2017 Accepted:25 July 2017 Published online: 11 August 2017
Ó
Springer Science+Business Media, LLC 2017ABSTRACT
Germanium nanostructures are prepared by electrochemical etching of n-type Sb-doped (100) oriented germanium (Ge) substrates with resistivity of 0.01 X cm. Ge substrates are etched in an electrochemical double cell containing hydrofluoric acid and ethanol solution at room temperature. Although the use of illumination source is essential for etching of an n-type semiconductor material, the influence of illumination source type on the germanium surface morphology has not yet been investigated. In this work, the illumination effect is studied by halogen lamp, white LED, 470- and 405-nm pulsed diode laser. It is demonstrated that different Ge surface morphologies such as nanocone, nanorod, nanoplate and nanopyramid are obtained using different illumination source. The current density, anodization time and pulsed laser power density effects on Ge nanopyramid are investigated in order to optimize anodization conditions. The most uniform and continuous Ge nanopyramid array is obtained at the current density of 30 mA/cm2 for 45 min under cathode side illumination with 470-nm pulsed diode laser. It is observed that the nanos-tructured Ge surfaces exhibit a broad photoluminescence band between 400 and 650 nm. Time-resolved fluorescence spectroscopy studies of electron transfer process between BODIPY dye and Ge nanostructures are reported. The obtained fluorescence lifetime data are analyzed in the light of the Marcus electron transfer theory to determine the conduction band energy level of nanostructured germanium substrates.
Address correspondence toE-mail: sabriyeacikgoz@kmu.edu.tr
DOI 10.1007/s10853-017-1434-6
ductor materials can be enhanced by reducing the size of the semiconductor to nanometer scale. Espe-cially, most research efforts have concentrated on designing silicon-based optoelectronic devices such as solar cell [1], chemical and biological sensor [2,3], photonic crystals [4], light emitting diode [5] and lithium-ion batteries [6] after the discovery of visible photoluminescence from nanostructured semicon-ductor surface.
Silicon and germanium are the widely used semi-conductors. There are several thousand papers about the fabrication of silicon nanostructures, structural and spectroscopy characterization and their device application. Although germanium has unique prop-erties like higher refractive index, higher dielectric constant, higher electron mobility, smaller band gap and larger effective Bohr radius as compared to sili-con, the number of research papers about nanos-tructured germanium does not exceed one hundred. Various methods had been successfully used to pre-pare germanium nanostructures such as electron beam evaporation [7], focused ion beam milling [8], molecular beam epitaxy [9], chemical vapor deposi-tion [10] and laser ablation [11]. Nanostructure growths by these all methods need a lot of time, a high vacuum or a special environment and subse-quent thermal annealing.
In this work, we used double-cell electrochemical etching technique in order to fabricate nanostruc-tured germanium surfaces. Over the last decades, this easy and low-cost method has been used for pro-ducing pores in germanium semiconductors but obtained structures are strongly non-uniform and irreproducible [12,13]. The effect of current density, etching time, acid solution type and its concentration on the structural properties of germanium surface is investigated [14–17]. However, the influence of the illumination light source on the surface morphology
Optical absorption in solar cells can be significantly increased by fabricating a continuous nanopyramid array which can manipulate light rays and enable extreme light trapping in active layer [18]. Han et al. [19] have demonstrated that Ge nanopyramid thin film coated on a silica glass substrate is capable of achieving nearly 100% absorption in the wavelength range from 500 to 800 nm. Riedel et al. [20] have prepared germanium nanopyramids on silicon sub-strates with different nanopyramid densities using chemical vapor deposition method to study the impact on contact angles, protein adsorption and cell behavior. Moreover, the nanostructured germanium surface can be used as an anode material for lithium-ion batteries due to its large specific capacity and good electrical conductivity [21,22].
On the other hand, dye-doped uniform germanium nanostructured surfaces can be a promising material for the development of new optoelectronic devices. However, the dynamics of fluorescence mechanism of the dye–Ge nanostructure has not yet been satis-factorily studied in the scientific literature. In this article, the photonic interactions between difluoro {2-[1-(3,5-dimethyl-2H-pyrrol-2-ylidene-N)ethyl]-3,5-dimethyl-1H-pyrrolato-N}boron (BODIPY) dye molecules and Ge nanostructures are also monitored by fluorescence lifetime imaging microscope (FLIM). It is observed that electron transfer reaction occurs at dye molecule–semiconductor interface. We espe-cially focus on the influence of the energy difference between the excited state energy of dye molecule and the conduction band energy of the semicon-ductor on electron transfer dynamics. The changes in the conduction band energy levels of Ge substrates due to the quantum confinement effect are deter-mined by combining the fluorescence lifetime data from the time-resolved measurement with Marcus electron transfer theory.
Materials and methods
Fabrication of germanium nanostructures
Nanostructured germanium samples are prepared by electrochemical etching of n-type Sb-doped (100) oriented germanium wafers with resistivity of 0.01 X cm. Ge wafers are etched in an electrochemical dou-ble cell with a HF:C2H5OH (1:3) electrolyte solution at room temperature. As shown in Fig.1, two equal acid cells are separated by Ge wafer and each cell contains a platinum electrode. Therefore, the current is forced to pass through the wafer during the etching process. Ge wafer is illuminated by halogen lamp, white LED, blue and violet pulsed diode laser head with wavelength 470 and 405 nm (Picoquant, LDH-D-C-470 and LDH-D-C-405). The effects of the illu-mination source, illuillu-mination side, current density, etching time, pulsed laser intensity and its wave-length on the obtained Ge nanostructures are inves-tigated. All experiments are performed at least twice to ensure reproducibility.
Functionalization of germanium
nanostructures with BODIPY dyes
To prepare germanium nanostructure with cova-lently attached dye molecules, the nanostructured substrate is firstly functionalized with 3-amino-propyltriethoxysilane (APTES) by immersing in a
solution containing 0.2 ml APTES and 10 ml toluene at 60 °C for 24 h. The modified substrate is washed in pure toluene three times for 2 min each to remove physically attached APTES molecules and dried in a clean oven at 110 °C for 30 min. Then, amino-func-tionalized substrate is treated with BODIPY (Sigma-Aldrich) dye solution at room temperature for 24 h. Dye molecules are covalently bound to the free ami-nes on the substrate through an amide linkage. After the substrate has been removed from the dye solu-tion, it is rinsed with methanol for 5 min to remove any excess dye and dried at room temperature.
Fluorescence lifetime imaging microscope
setup
Two-dimensional fluorescence intensity and lifetime images of BODIPY-attached Ge nanostructures are obtained by fluorescence lifetime imaging micro-scope (FLIM). A sketch of our home-made FLIM system is given in Fig.2. Time-correlated single-photon counting (TCSPC) technique is used to acquire fluorescence lifetime and intensity data simultaneously. Samples are excited by a pulsed 470-nm diode laser (LDH-C-D-470 Picoquant, GmbH) capable of producing 70-ps pulses with frequency of 80 MHz. The elliptical laser beam is coupled into a single-mode optical fiber (Thorlabs, S405-HP) to form a circular Gaussian beam. Two-dimensional scan of the sample is performed by a piezoelectric scanning
Figure 1 Double-cell electrochemical anodization setup.
stage (PI, P-733.2CD) and a digital piezo driver (PI, E-725.3CDA). Moreover, an extra long working dis-tance microscope objective (Nikon, ELWD 100X) is mounted to PIFOC piezo flexure objective scanner (PI, P-721.CDA) for the highest resolution imaging. In order to construct a confocal microscope system and reject the out of focus light, a 75-lm circular pinhole (Thorlabs, P75S) is placed in front of the photod-edector (SPAD, Micro Photon Devices). Data acqui-sition and analysis are performed using a Picoharp 300 (Picoquant, GmbH) time-correlated single-pho-ton counting instrument and SymPhoTime 64 (Pico-quant, GmbH) software. The fluorescence decay curves are analyzed in terms of multiexponential model
I tð Þ ¼X
i
Aiexpðt=siÞ ð1Þ
where Ai and si are the amplitude and fluorescence
lifetime of each component, respectively. The values of Ai and si are determined by minimizing v2. The
mean decay time is given by the following formula: hsi ¼ P iAis2i P iAisi ð2Þ
Results and discussion
Electrochemical etching method is used to fabricate germanium nanostructures with different shape and sizes. Germanium atoms dissolve in HF-based acid solution through divalent dissolution mechanism. When Ge wafer is immersed in HF solution, surface
of the germanium is passivated by hydrogen atoms. If the applied current density is sufficient, HF2-ions formed in the electrolyte solution interact with Ge atoms in the presence of holes and hydrogen atoms replaced by fluorine atoms. Thus, Ge atom removes from the bulk substrate as GeF4, and this molecule
reacts to GeF6- by binding two F ions in HF acid
solution. H2molecules are also generated during the anodization of Ge, and hydrogen gas bubbles are observed on the surface of the substrate. The chemi-cal reaction equation of the divalent dissolution of Ge is given by Eq. (3).
Ge þ 4HF2 þ hþ! GeF26 þ 2HF þ H2þ e ð3Þ
Positive charge carriers (holes) play an important role in the electrochemical etching process of the Ge substrate. The dissolution rate of Ge atoms strongly depends on the amount of holes at the semiconduc-tor–electrolyte interface. Moreover, a uniform hole distribution results in a uniform dissolution of the semiconductor atoms. Holes necessary for etching of n-type semiconductor can be generated by illumi-nating the semiconductor surface. The main purpose of this work is to demonstrate that the generated hole distribution and dissolution of atoms can be con-trolled by changing the optical properties of the illumination source.
The illumination source and illumination side effects on the surface morphology of germanium substrates are investigated. Firstly, the polished side of germanium substrate is placed through the plat-inum electrode which is electrically connected to anode, and this polished surface is illuminated with different light sources such as white LED, halogen
lamp, blue and violet pulsed diode lasers. The cur-rent passing through the germanium is kept constant at 50 mA/cm2 for 45 min. The field emission scan-ning electron microscope (FESEM) is used to char-acterize the surface morphology of germanium substrates and the obtained FESEM micrographs are given in Fig.3. It is observed that germanium nanorods in the size range of 50–100 nm are formed when germanium surface is illuminated by a halogen lamp. According to the FESEM image given in Fig.3a, these nanorods combine like pine needles on the surface of germanium substrates. When the ger-manium substrate is etched under illumination of a white LED, conical nanostructures with different sizes are distributed all over the entire etched area as shown in Fig.3b. Moreover, some rectangular pores are observed on the top of these nanodomes. This germanium surface can be accepted as a macrop-orous semiconductor structure because the sizes of these pores are bigger than 50 nm. Although white LED and halogen lamps are polychromatic and non-coherent sources, their PL spectra are different from each other. White LED PL spectrum contains more blue and yellow light. The tungsten filament of a halogen lamp produces yellowish white light with its
peak on the warm (red) side of the frequency spec-trum. Consequently, energy of photons emitted from the LED source is higher than halogen lamp. For hole generation on the surface of the semiconductor, the energy of the photons must be equal or greater than the band gap of the material. Figure 3c shows the surface morphology of a germanium wafer etched under blue (470 nm) pulsed diode laser. Using a monochromatic, coherent and pulsed light source leads to the growth of pyramidal nanostructures on the surface of germanium substrate. It is observed that the size distribution of the obtained nanopyra-mids is not uniform. The influence of the wavelength of pulsed laser is also investigated, and FESEM image of the Ge substrate etched under illumination of 405-nm pulsed diode laser is given in Fig.3d. Although the power densities and pulse duration of both laser sources are equal, the pyramidal nanos-tructures are not formed under 405-nm laser illumination.
Secondly, the polished side of germanium sub-strate is placed through the cathode electrode, and the polished surface is etched under illumination of halogen lamp, 470- and 405-nm pulsed diode lasers, respectively. Applied current density is 30 mA/cm2
Figure 3 FESEM images of germanium nanosurfaces etched from anode side by illuminatinga halogen lamp, b white LED, c 470-nm andd 405-nm pulsed diode laser. Allscale bars are 2 lm.
and the etching time is 45 min for all samples. FESEM images of fabricated germanium nanosurfaces are given in Fig.4. When germanium surface is illumi-nated by a halogen lamp, square-based germanium nanopyramids are formed on the substrate. As halo-gen lamp emits over a relatively broad spectral range and it contains photons of a lot of different energy values, the size distribution of the fabricated nanopyramids is not uniform. As shown in Fig.4b, more sharp nanopyramids and a uniform distribu-tion can be obtained by illuminating the germanium surface with 470-nm pulsed diode laser. However, the nanopyramids structures transform to nan-odomes when the energy of photons is increased by changing the wavelength of the pulsed diode laser. FESEM image of nanostructured germanium under 405-nm pulsed diode laser illumination is given in Fig.4c.
Bipolar electrochemical etching method is also used to fabricate different nanostructures on germa-nium surface. In this method, the polarity of anode and cathode electrodes is periodically changed in cycles of 5 min. The current density passing through the germanium substrate is kept constant at 50 mA/
cm2, and the total etching time is the 45 min. Illu-mination source effect is also studied for bipolar etching. FESEM images of the fabricated germanium surfaces by illuminating halogen lamp and 470-nm pulsed diode laser are given in Fig. 5a, b, respec-tively. In the case of halogen lamp illumination, square-like germanium nanoplates are produced in HF acid solution. When the polarity of the platinum electrodes is reversed, these thin germanium nano-plates are welded to each other and these aggregated plates have generated an entirely homogeneous morphology (Fig.5a). However, a uniform surface is not formed when germanium surface is etched under 470-nm pulse diode laser illumination. The aggre-gated germanium structures clearly seen on the sur-face (Fig.5b) are the cause of the surface roughness. The nanopyramid growth conditions are optimized to maintain uniform and continuous pyramid island on the germanium substrate. To observe the influence of applied current density on the surface morphol-ogy, germanium substrates are etched at different current densities such as 20, 25, 30 and 40 mA/cm2. Germanium substrates are illuminated by 470-nm pulsed diode laser during the etching process, and
Figure 4 FESEM images of germanium nanosurfaces etched from cathode side by illuminatinga halogen lamp, b 470- and c 405-nm pulsed diode laser. Allscale bars are 1 lm.
Figure 5 FESEM images of germanium nanosurfaces produced by bipolar electrochemical etching undera halogen lamp, b 470-nm pulsed diode laser illumination.
the etching time is kept constant at 45 min. FESEM images of etched germanium surfaces are given in Fig.6. Unfortunately, a linear relationship between the size of the nanopyramid and applied current density has not been established. Increasing current density gives rise to an increment in the amount of the dissolved germanium atoms. Because of the fact that germanium nanopyramids are in very close proximity to each other, every germanium atom leaving from the substrate causes an increase in the outer face of a pyramid and a decrease in the outer face of a neighboring pyramid. It is revealed that the most uniform and homogenous germanium nanopyramid island is fabricated when the applied current density is 30 mA/cm2. Moreover, an energy-dispersive X-ray spectroscopy (EDS) spectrum
recorded on the fabricated germanium nanopyramid is given in Fig.6e. It is clearly observed that the nanopyramids are mainly composed of the germa-nium substrate material. The peak for oxygen (O) is also noted. The average values of Ge and O elements in one nanopyramid are 97.86 and 2.14 wt%, respec-tively. The presence of small oxygen peak indicates the formation of oxidized layer.
To study the effect of etching time on the mor-phology, the reaction is continued for 15, 30, 45 and 90 min. FESEM images of the etched substrates at different etching time are given in Fig.7. Substrates are etched under illumination of 470-nm pulsed diode laser. The uniformity of germanium surface is improved by increasing etching time at constant current density of 30 mA/cm2. It is observed that the
Figure 6 FESEM images of germanium nanosurfaces etched from cathode side by applying current density of a 20, b 25, c 30, d 40 mA/cm2
ande EDS spectrum recorded on the fabricated germanium nanopyramid. Allscale bars are 500 nm.
most uniform germanium nanopyramid island is produced for etching time of 45 min. With elongated etching time, the surface morphology evolves from nanopyramids to nanodomes.
The influence of pulsed laser power on the growth of germanium nanopyramids is also investigated. Pulsed diode laser with a wavelength of 470 nm is used, and its pulse repetition rate and pulse duration are fixed at 80 MHz and 70 ps, respectively. The illumination area of the laser beam is approximately 8 mm2. Germanium substrates are exposed to three different laser irradiances such as 30, 40 and 50 mW/ cm2. Substrates are etched by applying current den-sity of 30 mA/cm2for 45 min. FESEM images of the fabricated germanium nanosurfaces are compared in Fig.8. It is revealed that square-based pyramidal nanostructures are formed on each of the substrates and the size of the nanopyramids increases for increasing laser power density. When laser power density is increased to 50 mW/cm2, the pyramidal nanostructured layers ruptured locally and the for-mation of square pores over structure with different sizes could be seen in Fig.8c.
In a bulk semiconductor material, electrons can be found valance or conduction energy bands. These wide and continuous energy bands are separated from each other by an energy gap Eg
. The
forbidden band gap energy values of silicon and germanium are 1.1 and 0.67 eV, respectively. There-fore, bulk silicon and germanium do not emit light in the visible region of the light spectrum. When the sizes of these semiconductor materials are reduced to nanometer, their photoluminescence properties sig-nificantly change due to quantum confinement effect. The number of allowed energy levels which can be occupied by a confined electron suddenly decreases, and the continuous energy bands transform to dis-crete energy bands. Consequently, the forbidden band gap value increases, and the nanostructured semiconductors are able to emit light in the visible region.
In this work, the photoluminescence properties of the nanostructured germanium surfaces produced by electrochemical etching are also investigated using fluorescence spectrometer (PTI, QuantaMaster 300). The germanium surfaces are excited by the 290-nm emission of a pulsed xenon light source. The photo-luminescence spectrum of germanium nanopyramids fabricated by etching of substrate under halogen lamp illumination from cathode side is given in Fig.9. This PL spectrum exhibits a broad band within the wavelength range 400–650 nm. The origins of this broad PL can be attributed to defects in the germa-nium dioxide (GeO2) matrix and nanostructures
Figure 7 FESEM images of germanium nanosurfaces etched from cathode side by applying current density of 30 mA/cm2fora 15, b 30, c 45 and d 90 min. Allscale bars are 1 lm.
formed in germanium material. In order to gain fur-ther insight into the photoluminescence of GeO2 layer, hydrogen peroxide (H2O2) solution is dripped out on the germanium surface and dried at room temperature. Subsequent treatment with H2O2leads to formation of a native germanium dioxide layer on germanium substrate. The photoluminescence spec-tra recorded before and after H2O2 treatment are compared in Fig.9. It is revealed that the intensity of the PL spectrum increased and its peak is shifted to 490 nm. This result provides direct evidence that the photoluminescence spectrum of GeO2 has a peak at 490 nm.
The cathode side illumination source, pulsed laser power density, etching current density and etching
time effects on the photoluminescence properties of germanium nanopyramids are analyzed. The varia-tions between the PL spectra are summarized in Fig.10. It is observed that nanostructured germa-nium surfaces exhibit a broad photoluminescence band between 400 and 650 nm. The reason behind the broadening of emission spectrum is the size fluctua-tions of nanopyramids. The influence of illumination source used during the etching process is firstly studied. As shown in Fig. 10a, using pulsed laser source results in a new photoluminescence peak at 540 nm. The observation of this new PL peak pro-vides direct evidence that the band gap energy of Ge nanostructures fabricated under laser illumination is smaller than that of Ge nanostructures formed under halogen lamp illumination. The effect of laser power density is demonstrated in Fig.10b. It is revealed that increasing laser power density causes an increment in the photoluminescence intensity. Applied current density and etching time effects are summarized in Fig.10c, d, respectively. For low current density and short etching time, the high density of GeO2 mole-cules formed on the germanium surface induced a significant localization effect resulting in strong photoluminescence peak at 490 nm. When current density and etching time are increased, these GeO2 molecules dissolve rapidly in HF acid solution and
Figure 9 PL spectra of nanostructured germanium surface before and after H2O2treatment.
their photoluminescence intensity starts to decrease. Consequently, the photoluminescence of nanostruc-tured germanium becomes more pronounced.
BODIPY dye molecules are covalently attached to nanostructured germanium surfaces having different shape, and the fluorescence lifetime of these dye molecules is analyzed using time-correlated single-photon technique. Electrons in the excited state of dye molecules can be transferred to the conduction band of the semiconductor material at dye–semi-conductor interface. According to the interfacial electron transfer theory developed by Marcus, the electron transfer rate kð ETÞ from a donor dye molecule
to a semiconducting surface is given by [23] kET¼ 2p h V 2 el 1 ffiffiffiffiffiffiffiffiffiffiffiffiffi 4kkBT p exp DG 0þ k 2 4kkBT " # ð4Þ
where h is the Planck constant divided by 2p, Velis the electronic coupling matrix element, kB is Boltz-mann’s constant, T is the temperature, DG0 is the
driving force and k is the total reorganization energy. The total reorganization energy of electron transfer process is the sum of two components as,
k¼ kiþ ko ð5Þ
where ki and ko are the inner and outer sphere
reor-ganization energy, respectively. The inner reorgani-zation energy can be taken as 0.3 eV, and the outer reorganization energy can be calculated using con-tinuum theory for a dye molecule at a semiconductor interface as [24,25] ko¼ e2 8pe0 1 D 1 n2 1 e 1 2R n2 sc n2 n2 scþ n2 1 n2 e2 sc e2 e2 scþ e2 1 e2 " # ( ) ð6Þ where e is the electron charge, nsc and esc are the
refractive index and dielectric constants of the semi-conductor nsc¼ 4:005 and esc¼ 16:2 for germanium
, n and e are the refractive index and dielectric
con-stants of the solvent
n ¼ 1:326 and e ¼ 31:95 for methanol
ð Þ [26], D is the
radius of the dye molecule and R is the distance between dye and surface. The spherical approach can be utilized to determine the radius of a BODIPY molecule [24]. By inserting the Avogadro’s number
NA
density q ¼ 1:5 g/cm 3of BODIPY into the follow-ing equation, 4 3pD 3 ¼ M NAq ð7Þ the radius of BODIPY is calculated as 4.12 A˚ . The outer reorganization energy is calculated as 0.54 eV with Eq.6. Thus, the total reorganization energy of BODIPY and Ge system becomes 0.84 eV.
The most important factor determining interfacial electron transfer rate is the driving force DG 0
dic-tated by the energy difference between the oxidation potential of donor molecule and reduction potential of acceptor. The driving force is given by Rehm– Weller equation [27]
DG0¼ e E½ Oxi:ð Þ ED Red:ð ÞA DE ð8Þ
where e is the unit electrical charge, EOxi:ð Þ andD
ERed:ð Þ are the oxidation and reduction potentials ofA
electron donor and acceptor, respectively. DE is the energy gap between ground and the first excited states of donor species. Oxidation and reduction potential of BODIPY dye molecules are determined as 1.12 and -1.22 V, respectively [28]. The reduction potential or conduction band edge of bulk Ge sub-strate is measured as -0.124 V [29]. The driving force for bulk germanium substrate and BODIPY dye
molecule is calculated as -1.10 eV using Eq.8. Because of the fact that the calculated driving force is bigger than reorganization energy of BODIPY–Ge system, the electron transfer process under such conditions can be correlated to Marcus inverted region where the electron injection rate increases with decreasing driving force.
In this work, electron transfer process between BODIPY and germanium structures such as micropyramid, nanopyramid, nanoplate, nanocone and nanorod is monitored using confocal home-made fluorescence lifetime imaging microscope (FLIM) system. FLIM technique provides high-resolution spatial lifetime image of dye molecules on germa-nium nanostructures. The obtained two-dimensional fluorescence intensity and lifetime maps are given in Fig.11. The color contrast of FLIM image is generated by the differences in the lifetime of individual fluo-rophores in each pixel using a continuous pseudo-color scale ranging from 2 to 3.6 ns. In order to calculate electron transfer rate, the fluorescence life-time of a free BODIPY molecule and a BODIPY molecule covalently bound to Ge nanostructures must be compared. BODIPY molecule is deposited onto a glass substrate, and its average fluorescence lifetime is measured as 4.593 ns. Decay parameters of BODIPY molecules attached to bulk, micropyramid,
Figure 11 Confocal FLIM images of Gea micropyramid, b nanopyramid, c nanoplate, d nanocone, e nanorod and f chemical structure of a BODIPY dye molecule. The size of FLIM andinset SEM images are 60 60 lm and 2 2 lm, respectively.
nanopyramid, nanocone, nanoplate and nanorod Ge substrates are summarized in Table1. It is revealed that the fluorescence lifetime of BODIPY dye mole-cules displays a strong dependence on the geometry of nanostructured surface.
A schematic representation of the interfacial elec-tron transfer process between BODIPY and Ge nanostructures is shown in Fig.12. Oxidation and reduction potentials are drawn relative to the normal hydrogen electrode (NHE) given on the right. When the size of a semiconductor material is reduced to nanometer scales, its conduction band energy (EC) shifts to higher energy level due to quantum con-finement effect. Thus, the energy difference between the lowest unoccupied molecular orbital (LUMO) energy of donor dye molecule and the conduction band energy level of nanostructured semiconductor
DG0
decreases, and effective electron injection into the conduction band of the semiconductor is highly enhanced. These nonradiative transitions cause a considerable reduction in the measured fluorescence lifetime of BODIPY dye molecules. Electron transfer rate can be experimentally determined by probing the changes in the fluorescence lifetime. Electron
transfer rates are calculated using the following equation kET¼ 1 sGeþBODIPY 1 sBODIPY ð9Þ where sBODIPY and sBODIPYþGe are the fluorescence
lifetimes of the BODIPY in the absence and presence of the Ge nanostructures, respectively. The calculated electron transfer rates and related driving forces are given in Table 2. These results indicate that the BODIPY/Ge nanoplate system exhibits the highest electron transfer rate of 20.3 9 107s-1.
It is observed that the fluorescence lifetime of BODIPY dye molecules on Ge nanocone and nanorod
Figure 12 Diagram of the interfacial electron transfer mechanism between BODIPY and Ge.
Table 2 Calculated electron transfer rates from BODIPY dye molecules to germanium nanostructures, driving forces and con-duction band energy levels
Sample sðnsÞ kET(s-1) DG0ðeVÞ ECðeVÞ
Bulk Ge 3.120 10.2 9 107 -1.10 -4.38 Micropyramid 3.112 10.4 9 107 -1.09 -4.37 Nanopyramid 2.602 16.6 9 107 -1.00 -4.28 Nanoplate 2.376 20.3 9 107 -0.94 -4.22
structures is bigger than the free dye molecules. According to the Marcus electron transfer theory, the energy of the LUMO of the dye molecule must be higher than the energy of ECin the Ge nanostructures for an efficient electron injection from dye. Therefore, the conduction band energy level that exceeds the LUMO of dye molecule can be adduced as a reason for the increased fluorescence lifetime observed in Ge nanocone and nanorod structures. Consequently, electron cannot be transferred from BODIPY to these substrates. On the other hand, this long lifetime can be attributed to a back electron transfer from the n-type semiconductor to dye molecules [30].
The reduction in the measured fluorescence lifetime from dye-modified Ge nanostructures provides a convenient means of determining the conduction band energy level of these nanostructures. The theoretical solid curve in Fig.13for kETversus DG0(using Eq.4) is
generated by simply giving arbitrary values to DG0
from 0 to 2 eV. Electronic coupling matrix (Vel) value is taken as 0.11 meV. Its experimental value is deter-mined by inserting kET and DG0 values of bulk
ger-manium substrate in Eq.4. The energy difference between LUMO of BODIPY dye and the conduction energy level of Ge nanostructures DG 0is determined
using theoretical curve given in Fig.13. The driving forces and the conduction energy levels ðECÞ of Ge
nanostructures are summarized in Table2.
Conclusion
In this work, nanostructured germanium surfaces are produced by double-cell electrochemical etching method. It is observed that illumination light source
is an important tool to form different nanostructures on the anode side of Ge substrates with controlled morphology. Using a monochromatic, coherent and pulsed light source leads to the formation of square-based pyramidal nanostructures on the surface of germanium substrate. Changes in the current density, etching time and laser power density drive to varia-tions in the size of the nanopyramids. The most uniform and continuous nanopyramid array is fab-ricated by applying 30 mA/cm2 current density under laser fluence of 30 mW/cm2for 45 min. Visible range photoluminescence properties of Ge surfaces are analyzed, and a broad peak between 400 and 650 nm is observed. This broad peak is attributed to the radiative recombination of excitons confined in nanostructured germanium and GeO2molecules. The photoemission peak corresponding to GeO2molecule is clearly distinguished by H2O2 treatment. More-over, an additional peak at about 540 nm is observed for nanostructured Ge surfaces prepared by illumi-nating pulsed laser source. These continuous Ge nanopyramid arrays and their photoluminescence properties may lead to the next generation of high efficient optoelectronic devices. Moreover, photonic interaction between BODIPY dye molecules and Ge nanostructures are investigated. FLIM technique and interfacial electron transfer theory are successfully combined to monitor Ge nanostructures and obtain the conduction band energy level of nanostructured Ge surfaces.
Acknowledgements
This work was supported by TUBITAK under Grant Number 114F451 and Karamanog˘lu Mehmetbey University Research Fund under Grant Number 16-M-15.
References
[1] Oh J, Yuan HC, Branz HM (2012) An 18.2% efficient black silicon solar cell achieved through control of carrier recom-bination in nanostructures. Nat Nanotechnol 7:743–748 [2] Sainato M, Strambini LM, Rella S, Mazzotta E, Barillaro G
(2015) Sub-parts per million NO2chemi-transistor sensors
based on composite porous silicon/gold nanostructures pre-pared by metal assisted etching. ACS Appl Mater Interfaces 7:7136–7145
Figure 13 Plot of BODIPY (donor) and all Ge surfaces (acceptor) electron transfer data.
Homewood KP (2001) An efficient room-temperature sili-con-based light-emitting diode. Nature 410:192–194 [6] Wang W, Favors Z, Ionescu R, Ye R, Bay HH, Ozkan M,
Ozkan CS (2015) Monodisperse porous silicon spheres as anode materials for lithium ion batteries. Sci Rep 5:8781–8786
[7] Kumar RR, Rao KN, Phani AR (2011) Growth and char-acterization of germanium nanowires on a flexible aluminum substrate by electron beam evaporation. Appl Nanosci 1(4):211–217
[8] Kolibal M, Matlocha T, Vystavel T, Sikola T (2011) Low energy focused ion beam milling of silicon and germanium nanostructures. Nanotechnology 22:105304–105311 [9] Pchelyakova OP, Bolkhovityanova YB, Dvurechenskiia AV,
Nikiforova AI, Yakimova AI, Voigtlander B (2015) Molec-ular beam epitaxy of silicon–germanium nanostructures. Thin Solid Films 367:75–84
[10] Dailey JW, Taraci J, Clement T, Smith DJ, Drucker J, Picraux ST (2004) Vapor liquid solid growth of germanium nanostructures on silicon. J Appl Phys 96:7556–7567 [11] Seo MA, Kim DS, Kim HS, Choi DS, Jeoung SC (2006)
Formation of photoluminescent germanium nanostructures by femtosecond laser processing on bulk germanium: role of ambient gases. Opt Express 14(11):4908–4914
[12] Flamand G, Poortmans J, Dessein K (2005) Formation of porous Ge using HF-based electrolytes. Phys Status Solidi (c) 2:3243–3247
[13] Fang C, Fo¨ll H, Carstensen J (2006) Electrochemical pore etching in germanium. J Electroanal Chem 589:259–288 [14] Kartopu G, Sapelkin AV, Karavanskii VA, Serincan U, Turan
R (2008) Structural and optical properties of porous nanocrystalline Ge. J Appl Phys 103:113518–113524 [15] Rojas EG, Hensen J, Carstensen J, Fo¨ll H, Brendel R (2011)
Porous germanium layers by electrochemical etching for layer transfer processes of high-efficiency multi-junction solar cells. ECS Trans 33(17):95–102
[16] Tutashkonkoa S, Boucherif A, Nychyporuk T, Kaminski-Cachopo A, Are`s R, Lemiti M, Aimez V (2013) Mesoporous germanium formed by bipolar electrochemical etching. Electrochim Acta 88:256–262
(2015) Germanium nanopyramid arrays showing near 100% absorption in the visible regime. Nano Res 8(7):2216–2222 [20] Riedel M, Mu¨ller B, Wintermantel E (2001) Protein adsorption and monocyte activation on germanium nanopyramids. Biomaterials 22:2307–2316
[21] Li X, Yang Z, Fu Y, Qiao L, Li D, Yue H, He D (2015) Germanium anode with excellent lithium storage perfor-mance in a germanium/lithium–cobalt oxide lithium-ion battery. ACS Nano 9(2):1858–1867
[22] Hwang J, Jo C, Kim MG, Chun J, Lim E, Kim S, Jeong S, Kim Y, Lee J (2015) Mesoporous Ge/GeO2/carbon
lithium-ion battery anodes with high capacity and high reversibility. ACS Nano 9(5):5299–5309
[23] Gao YQ, Marcus RA (2000) On the theory of electron transfer reactions at semiconductor/liquid interfaces. II. A free electron model. J Chem Phys 113:6351–6359
[24] Williams RM, Koeberg M, Lawson JM, An YZ, Rubin Y, Paddon-Row MN, Verhoeven JW (1996) Photoinduced electron transfer to C60 across extended 3- and 11-bond hydrocarbon bridges: creation of a long-lived charge-sepa-rated state. J Org Chem 61:5055–5062
[25] Darius K, Michael SF, Harry BG, Jay RW (2001) Electron transfer dynamics in nanocrystalline titanium dioxide solar cells sensitized with ruthenium or osmium polypyridyl complexes. J Phys Chem B 105:392–403
[26] Vyas AD, Rana VA, Gadani DH (2011) Dielectric properties of mixtures of some rigid polar molecules with some pri-mary. Indian J Pure Appl Phys 49:277–283
[27] Rehm D, Weller A (1970) Kinetics of fluorescence quenching by electron and H-atom transfer. Isr J Chem 8:259–271
[28] Nepomnyashchii AB, Bard AJ (2012) Electrochemistry and electrogenerated chemiluminescence of BODIPY dyes. Acc Chem Res 45(11):1844–1853
[29] Porter LA, Choi HC, Ribbe AE, Buriak JM (2002) Con-trolled electroless deposition of noble metal nanoparticle films on germanium surfaces. Nano Lett 2:1067–1071 [30] Tvrdy K, Frantsuzovc PA, Kamat PV (2011) Photoinduced
electron transfer from semiconductor quantum dots to metal oxide nanoparticles. PNAS 108:29–34
