A Spectroelectrochemical Method for Locating Fluorescence Trap

Jan 21, 2013 - Role of size and defects in ultrafast broadband emission dynamics of ZnO nanostructures. Kannatassen Appavoo , Mingzhao Liu , Matthew Y...
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A Spectroelectrochemical Method for Locating Fluorescence Trap States in Nanoparticles and Quantum Dots T. Jesper Jacobsson and Tomas Edvinsson* Department of Chemistry - Ångström Laboratory, Uppsala University, Box 538, 75121 Uppsala, Sweden S Supporting Information *

ABSTRACT: We here devise an electrochemical method for determining the absolute energetic position of trap levels involved in fluorescence. The method utilizes potentiostatic control of the Fermi level in the material, and thereby also the electronic population of the energy states involved in the fluorescence. The method is especially useful for nanoparticle semiconductor electrodes. Here we exemplify the method by determining the position of the trap levels involved in the green fluorescence in thin films of ZnO quantum dots. The exact mechanism and the absolute positions of these states have been debated in the literature. Here we show that the visible fluorescence is caused by a transition from energy levels slightly below the conduction band edge to a deep trap within the band gap. We further pinpoint the location of the upper trap level to be at 0.35 ± 0.03 eV below the conduction band edge. Particles between 5 and 8 nm in diameter have been analyzed, which is in the quantum confined region of ZnO. We also show that the position of the upper trap level shifts with the size of the quantum dots in the same way as the conduction band.

1. INTRODUCTION Fluorescence is a seemingly familiar and well-known phenomenon. It is seen in everything from common lightning applications, natural minerals, tonic water, and discotheques on party nights. The term fluorescence has been around since the 19th century,1 and a basic understanding of the general mechanism is considered as common knowledge and is described in any standard textbook of chemistry. A closer look under this fluorescent facade of general understanding does however reveal a large body of elusive details. In every fluorescent material system, the emitted light emerges from a transition between two energy states whose nature, spatial, and energetic location are not evident from the fluorescence itself. For some systems, quite a lot is known, but for others, we hardly know more than that they fluoresce. A deeper understanding of the nature of the trap states involved in the process does not only give insight into the inner structure of the material but may also be valuable in the design and optimization of devices utilizing the fluorescence. This could for example be in light emitting diodes and various analytical techniques. There are also occasions where effective quenching of the fluorescence by passivation of the trap states is desired. For nanoparticles, the nature of trap states is in many cases also directly related to the properties of the surface and information on these states is thus of significant importance both for a fundamental understanding as well as for applications of systems where the surface to bulk ratio is very high. In this paper, we describe an electrochemical method that can pinpoint the energetic positions of the conducting trap levels involved in the fluorescence from dense to quite nanoporous semiconductor electrodes. Electrodes with nano© 2013 American Chemical Society

particle assemblies are found in a plethora of applications such as in batteries, dye sensitized solar cells,2 gas sensors, and catalytic and photocatalytic applications.3 With this method, it is possible to change the chemical environment in order to analyze the surface nature of the trap states which are valuable, as they are different depending on the application in focus. Different techniques for locating trap levels are found in the literature, like transient spectroscopy,4 deep level transient spectroscopy, DLTS,5 and theoretical calculations. DLTS is a popular method and works excellent for silicon. It does however demand the presence of a space charge layer, which is ill-defined and in many cases not present in nanoporous electrodes. Theoretical computations on the other hand suffer from an idealized description of the material and the need of experimental verification to be certified. The method devised here is based on potentiostatic control of the Fermi level in the electrode, by which population of trap levels is controlled. This either enhances or quenches the fluorescence depending on the nature of the energy states involved. Since only electronically accessible states can be populated, the method solely probes the conductive states. These are of particularly importance, as they dictate the initial charge transfer into and out from the electrode surface. They can give rise to leakage currents and act as recombination centers, as well as improve device performance by charge balancing at interfaces and as charge reservoirs in catalytic processes. The obtained response also gives information Received: December 4, 2012 Revised: January 15, 2013 Published: January 21, 2013 5497

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to obtain good statistics, an average of over 100 consecutive spectra was done. The fluorescence is measured with the same detector as the absorption, but the integration time is increased by a factor of 1000. The detector is perpendicular to a UV-led lamp which is fed with 250 mA and has a maximum intensity at 365 nm. The UV source corresponds rather well with the band gap energies of the larger particles, and the width of the emission is large enough to overlap with the absorption spectra for all the investigated particles. The spectrum from the UV-led is given in the Supporting Information. The electrochemical measurements were performed with a CH Instrument model 760 C. Potentials are measured with respect to a Ag/AgCl reference electrode, which is shifted 0.197 V with respect to the normal hydrogen electrode (NHE). The electrolyte consisted of 0.5 M Na2SO4 in water, and a Pt-wire was used as a counter electrode. The fluorescence is measured as a function of applied potential. The potential is scanned between 1.6 and −1.1 V vs Ag/AgCl with a scan rate of 1 mV/s. One spectrum is measured every tenth second during the potential scan, corresponding to 100 spectra per volt. A photo of the experimental setup is given in Figure 1. The whole setup is covered with a black box during measurements to shield it from stray light.

concerning the kinetics of the processes involved. The method should in principle be applicable to any nanoporous semiconductor that shows fluorescence, as long as charge carrier limitations do not limit the population control. Here the method is applied to electrodes of ZnO quantum dots, which we have studied quite extensively before.6−9 ZnO is known to show fluorescence in both the UV region as well as in the yellow-green range,10,11 but fluorescence has also been reported for other wavelength regions.12,13 Here we concentrate on the green fluorescence for which the mechanistic origin and the energetic positions of the involved states have been under some debate in the literature. Various different mechanisms have been suggested and the fluorescence has been assigned to defects originating in both zinc vacancies, zinc interstitials,14 antisite oxygen,15 oxygen vacancies,16 and copper impurities.17 It is however clear that the nature of the surface has a large impact on the green emission. Using quantum mechanical calculations, the positions of these trap states have been calculated to be in all regions of the band gap15,18−20 for bulk material and selected surfaces. For nanoparticles, with surface induced trap states possible in many surfaces and interfaces between surfaces, representative quantum mechanical models are even more problematic. Here we show that the green fluorescence is caused by a transition from a level slightly below the conduction band edge to a deep trap level within the band gap. We pinpoint the energetic positions of these levels on an absolute scale, and discuss their nature based on their energetics and behavior in a changing environment. Finally, we show how the position of these levels depends on particle size in the quantum confined region of ZnO.

2. METHODS 2.1. Synthesis. ZnO nanoparticles were synthesized by a wet chemical method based on hydrolysis in alkaline zinc acetate solution. The synthesis is based on work of Meulekamp21 and Spanhel et al.,22 and the details are found in a previous study.6 As a short description, 2.5 mmol of Zn(OAc)2·2H2O is dissolved in 25 mL of boiling ethanol under vigorous stirring for approximately 1 min. The solution is subsequently cooled to room temperature and mixed with 3.5 mmol of LiOH·H2O dissolved in 25 mL of ethanol. When the two solutions are mixed, ZnO quantum dots begin to nucleate and gro,w which can be monitored by measuring the band gap shift with UV−vis spectroscopy.6,8 At certain times, a small volume of solution is extracted in order to make films of particles with distinct sizes. A 2.5 mL portion of reaction solution is then mixed with approximately 5 mL of hexane, which induces particle agglomeration and precipitation. In order to increase the speed of the sedimentation, the solution is centrifuged at 5000 rpm for 5 min. The particles were then redispersed in one drop of methanol and ultrasonicated for 3 min, after which they were doctor bladed on substrates of conductive glass. Smooth and transparent films were then formed. The substrates used for film deposition were fluorine doped SnO2 (FTO) Pilkinton TEC 8, which were used in order to enable combined electrochemical and transient absorption measurements. 2.2. Measurements and Characterization. The UV−vis absorption measurements were performed on an Ocean Optics spectrophotometer HR-2000+ with deuterium and halogen lamps. In all measurements, a full spectrum from 190 to 1100 nm with 2048 evenly distributed points was sampled. In order

Figure 1. Experimental setup while measuring potential dependent fluorescence. To the left is the UV-led. The film is in the electrolyte and in approximate 45° angle with respect to both the UV-led and the detector which can be discerned at the back of the picture. The reference electrode and the counter electrode are seen at the right side in the glass vessel.

3. THEORY If a potentiostat is utilized to apply a potential on an electrode, the Fermi level in the electrode will equal the applied potential under conditions detailed below. When such an electrode is composed of a semiconductor, there will be a driving force for populating energy levels below the applied potential with electrons from the potentiostat. For energy levels above the applied potential, no such driving force exists. If the applied potential is such that the Fermi level in the material is higher (more negative with respect to NHE), than the donor levels involved in the fluorescence there will thus be an influx of electrons into these levels from the potentiostat. The increased electronic population of these levels increases the probability for a radiative recombination with a lower acceptor state, thereby increasing the effective quantum yield of the fluorescence. 5498

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Figure 2. (a) Situation where the Fermi level, or applied potential, is below the energy levels from which electrons recombine with lower lying holes to give fluorescence. In the figure, (1) is the absorption creating an electron−hole pair, (2) is radiationless relaxation into a lower lying energy level, and (3) is recombination of an electron in this level with a hole causing fluorescence. (b) The potential is here changed to a more negative level, getting the Fermi level to match the donor level for the electrons. This causes an influx of electrons into these levels from the potentiostat (5), which increases the radiative recombination (4).

Figure 3. The probability distribution Pn of electrons in a steady-state situation for a situation where (a) the kinetics for the transport of electrons (ktrans) is faster than the kinetics of recombination (krec) and (b) the kinetics for the transport of electrons is slower than the kinetics of recombination. The diffusion length, Lp, expressed as a function of the effective diffusion (Dp) and characteristic decay time (τp) of the minority carrier is also marked.

Measuring fluorescence as a function of applied potential and noticing where the fluorescence begins to increase can then give a fairly accurate measure of where the trap levels are located. Performing this in a three-electrode setup gives the energetic position of the trap levels relative to the known level of the reference electrode. A sketch of the concept is given in Figure 2. The chemical environment can easily be changed by varying the electrolyte which can be utilized to analyze the surface nature of the trap states. To relate the position of the trap levels with respect to the band edges, the position of these needs to be known. Several methods for determining the band edge positions exist where one of the more efficient ones utilizes a potential analogue to the Burstein−Moss effect together with potential dependent absorption measurements.8 For ZnO quantum dots, the absolute positions of the band edges are known from such measurements performed in a previous study.8 The description above gives an intuitive and qualitative framework for the model used in this paper. The picture is however slightly more complex, and the population of energy states and the associated electron concentration, Pn, is essentially a statistical property. In the view of donor and acceptor notation, the electron concentrations and its contributions to the Fermi level, Ef, can as a reasonable good approximation be described by the Fermi−Dirac distribution, f(E) = 1/(1 + exp((E − Ef)/kBT)), which gives the occupation probability, f, of an orbital at energy E in an ideal electron gas at

thermal equilibrium, where kB is Boltzmann’s constant and T the absolute temperature. A system under illumination and potentiostatic control is strictly not in equilibrium due to the incoming photon flux and the increased electron population under these conditions. The situation will instead be a steady-state condition concerning the generation and losses of photogenerated charge carriers. The Fermi level would then instead be a quasi-Fermi level valid only as long as the steady-state condition is valid. This situation is illustrated in Figure 3 and simply verified experimentally by the occurrence of a constant photovoltage or the occurrence of a Burstein−Moss shift.8 The probability for occupancy of a donor states (DS) with respect to the quasi-Fermi level under steadystate conditions is then given by eq 1. fquasi (E DS) =

1 (E DS − E F)/ kT

1+e

(1)

The probability of occupancy of an orbital in a donor state is by definition f(EDS) = nDS/NDS, where nDS is the effective number of donor state electrons and NDS is the total density of donor states. The total occupation is given by multiplying by the degeneracy of the donor state or equivalent to the density of states. An increase in the Fermi level in the system to the donor state energy, that is, EF → EDS, gives that half of all the donor states will be populated, and in a doubly degenerated spin orbital, this means that one of the spin states will be occupied. 5499

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For a Fermi level above the energy of the donor states, the situation can be approximated by Boltzmann statistics, now with Fermi energy above the states, that is, −(EDS − EF) ≫ kT, giving nDS = NDS(1 − e−(EDS − EF)/ kT )

(2)

which is an exponential increase in the probability for population of the donor states when the Fermi level is above the donor energy level. The increase in population of donor states will result in a strong increase in the fluorescence as long as there are available acceptor states (holes). The acceptor states (AS) are described analogously to eqs 1 and 2 with EAS instead. As long as there are available acceptor levels, an increase in the fluorescence transitions under steady-state illumination and a successive increase in Fermi level must thus come from the population of donor levels. The expected experimental outcome based on this reasoning in an experiment where the potential is decreased from a higher level (the Fermi level is raised) would be a rather constant fluorescence until the potential reaches the lowest lying donor states. The fluorescence should then increase, and the onset of this increase will mark the lowest lying states. The fluorescence should then increase until the potential reaches so high that essentially all the donor states are populated where after it should level off at a higher value. The onset of the increase in fluorescence thus gives the first signs of accessible donor states, whereas the width of the fluorescence increase gives an indication of the distribution of the donor states and the position of the maximum in their density of states. In a real system, there will also be a spatial dependence of the electron concentration. In a steady-state situation where there are non-negligible losses at the electrolyte interface, the density of electrons in the donor states and the conduction band will be spatially dependent, as illustrated in Figure 3. The figure illustrates the condition of a dense, flat electrode, where w represent the bulk and Δw the region of distinctly nonuniform electron density. Another commonly encountered way of illustrating this condition is by introducing a band bending. For donor states located within the Δw region, a certain overpotential from the potentiostat is thus needed for populating these states and it can be expected that the width of the potential window where the fluorescence changes is larger in such a situation. Here, one can choose a very inert electrolyte, suppressing the recombination process to ensure that the overpotential is very low. The onset of the fluorescence increase is however unaffected by this complication. The method for measuring the absolute positions of the fluorescence donor states from the onset is thus generally applicable also in the case of nanoporous systems where the total surface area is large, since at least the part in contact with the Fermi level increased electrode will be at the potential level applied. A necessary condition for the method to work is that the population and depopulation of the donor states are within the characteristic time frame of the potential update time (kpot). This requires that the transport kinetics (ktrans1) of electrons to the donor states are faster than the losses in terms of recombination to the surrounding system for at least a small part of the system and thus give a measurable fluorescence increase, as discussed in context with Figure 3. In Figure 4, a schematic picture of the involved kinetic processes in the system is shown.

Figure 4. Schematics of the kinetics involved for the different possible electronic transitions.

The system is illuminated which is a process that in many semiconductors improves the conductivity via the photoconductive effect, and intrinsically improves the transport factor ktrans. Under illumination, the Fermi level in the semiconductor system increases due to photoinduced charge carriers (electrons) in the conduction band (CB). After a short time, the system is adjusted into a steady-state condition where the increase in CB electrons is balanced by relaxation into lower lying trap states (krel) and by losses via recombination to the electrolyte (krec), either directly from the CB or via the trap states, as illustrated in Figure 4. An obvious precondition for fluorescence to occur is of course that the nonradiative transition rate (knonrad) from the fluorescence donor states is slower, or at least not much faster, than the radiative process (krad). Under the reasonable condition that the conduction band edge and donor state levels are not moving under a fixed electrolyte and fixed particle size, the increased Fermi level by the potentiostat can populate conductive trap states and thus increase nDS. The theory described here assumes that the deeper acceptor levels are not filled by the potentiostat, which is equivalent to state that ktrans2 in Figure 4 is a slow process compared to the other processes involved. For ZnO, that is indeed the case, but that is not necessarily true in other material systems. If ktrans2 is fast, the method can still be used in a similar fashion. Instead of measuring an increase of fluorescence for potentials lower than the donor states, a decrease in fluorescence would be detected for potentials more negative than the acceptor states, as these are populated with electrons and instead lower the probability for a radiative transition of an electron into these states.

4. RESULTS Previous analysis of these ZnO quantum dots with XRD,6 absorption,8 and Raman spectroscopy9 showed relatively isotropical ZnO particles. For the details of the characterization of the particles and the films, we refer to these articles. From absorption measurements, the band gaps of the particles are given by plotting the square of the absorption versus photon energy and extrapolated down to zero absorption. The optical band gap has previously been related to the particle diameter6 which enables a determination of the particle size from absorption measurements. Here electrodes with seven different quantum dot sizes are investigated with particle diameters ranging from 5 to 8 nm, which are in the domain of electronic quantum confinement in ZnO. The absorption data and size determination are described in more detail in the Supporting Information. 5500

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Figure 5. Fluorescence as a function of wavelength and applied potential for a representative sample of particles 7.4 nm in diameter. (a) Fluorescence at certain applied voltages given with respect to NHE. (b) The three-dimensional version of part a. (c) The fluorescence data in the form of a couture plot.

Figure 6. (a) Maximum fluorescence as a function of applied potential for a representative sample. The data corresponds to the ridge in Figure 5b. In the figure, also the position of the conduction band edge and the flat band potential is given. (b) Maximum fluorescence as a function of applied potential during a cycling potential sweep, demonstrating that the process is reversible. The vertical line represents the turning point of the potential sweep. (c) Maximum fluorescence as a function of applied potential compared to the photocurrent in the potential sweep. The vertical line represents the maximum in fluorescence intensity.

The fluorescence as a function of wavelength and applied potential is given in Figure 5 for a representative sample with 7.4 nm particles. The dominant feature is a yellow-green fluorescence, also seen by the naked eye in Figure 1. During the potential scan, the fluorescence is rather constant with respect to both wavelength and intensity down to around −0.3 V vs NHE, where a sharp increase in fluorescence intensity occurs. At even more negative potentials, the fluorescence drops to a lower level. From the data in Figure 5, the maximum fluorescence, corresponding to the ridge in Figure 5b, is extracted and given in Figure 6a. By measuring the optical absorption as a function of applied potential and utilizing the Burstein−Moss effect, the absolute position of the conduction band edge can be determined, as demonstrated in a previous article.8 The absolute energetic position of the conduction band edge as well as the flat band potential and how it depends on particle size is thus known by using that method. This enables a direct comparison of the position of the markedly increased fluorescence to the positions of the band edges, as demonstrated in Figure 6a. The marked increased in fluorescence evidently occurs at potentials below (less negative vs NHE) the conduction band edge, revealing the presence of electronically accessible trap states 0.35 eV below the conduction band edge. It is further noticed that the fluorescence is rather constant for potentials higher (more positive vs NHE) than the observed peak. The optical properties are fully reversible with respect to potential. This is illustrated in Figure 6b, where the fluorescence is measured during a reversible potential sweep.

Overlaid on this data, there is a slight decrease of fluorescence from 7500 to 7000 counts over time. From Figure 5 and 6, one can see a marked increase in fluorescence intensity at 0.35 eV below the conduction band edge. This is from the population of trap states with subsequent increase in probability for radiative and nonradiative recombination. Since (krad ≫ knonrad) and (krad > krec), a relative strong increase in fluorescence intensity is seen. Even for systems where the fluorescence is kinetically competing with the recombination processes (krad ≈ krec), and the transport is slow in comparison to the recombination (ktrap < krec), the method is applicable, since it is enough to see any small increase in fluorescence to pinpoint the position of the conductive donor trap states. These particle films have previously been demonstrated to show both photocatalytic degradation of organic dyes as well as the oxidation of water into O2(g) under AM 1.5 illumination.7 In Figure 6c, the photocurrent under the UV-illumination is compared to the maximum of the fluorescence. The fluorescence peak potential coincides with the point where the photocurrent changes polarity and water no longer is oxidized. For more positive potentials, there is a flow of electrons from the electrolyte into the film and further into the back contact due to water oxidation. In the interpretation of the data in Figures 5 and 6, an explanation is needed for the drastic drop in fluorescence at potentials more negative than that for the fluorescence maximum. Different mechanisms could affect the measured fluorescence when the potential reaches these levels. Several seemingly plausible explanations can however be excluded. The 5501

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short lifetime for this to be the case. It is enough for the step forming the final deep trap to which the fluorescence generating electron relaxes to be a fast process. ZnO is known to be used in electroluminescence devices.30−33 These are generally based on pn-junctions, ensuring the presence of both electrons and holes. Here, this is not the case and a potential scan in the dark does not give rise to any electroluminescence. This is not particularly surprising, since even if electrons can be inserted into the electrode by the potentiostat there will be a very limited amount of holes for the electrons to combine with. In summary, applying the described method, we demonstrate the visible fluorescence to originate from a transition from trap levels centered at 0.35 V below the conduction band edge down to acceptor levels located 2.65 eV below the conduction band. A graphical summary of the findings is given in Figure 7.

surrounding solvent in the system consists of water, and at the location of the fluorescence peak, the photo-oxidation of water stops, which can be attributed to the change in the electric field and charge balance at the semiconductor−electrolyte interface. This would however increase the concentration of holes in the semiconductor and should thus increase the probability for a radiative recombination, and is thus not a cause of the decrease in fluorescence. Another possibility is based on the fact that at more negative potential the Burstein−Moss effect becomes significant.8 This has the effect of increasing the apparent band gap and results in accessible conduction band levels only at more negative potentials, which would decrease the absorption and thus also the fluorescence. We have demonstrated the absorption coefficient to decrease at more negative potentials which should have the same effect. These effects are quantified in the Supporting Information, and it turns out that they only present themselves at potentials significantly more negative than the fluorescence peak, and they can thus not be the cause of the decrease. The relevant plots for these situations are given in the Supporting Information. The decrease must instead be attributed to changes of the surface properties as a consequence of the reversal in the direction of the photocurrent in the electrode. There are several studies demonstrating the surface properties of ZnO to be of great importance for the extent of the visible fluorescence.23−27 With increased potential, water oxidation ceases and photocurrent is reversed, resulting in changes of the hydroxylated state of the surface. The photocurrent under current experimental conditions is small and on the order of 10 μA/ cm2, but the change from oxidizing to reducing water at the electrolyte interface will raise the local pH at the very surface of the particles, not enough for a high alkaline situation but enough to decrease the oxygen termination in favor of hydroxide termination at the surface. Van Dijken et al. have in a series of papers proposed a hypothesis where the green fluorescence is caused by a 10,28,29 transition to a double charge oxygen vacancy (V•• O ). The most common defect in ZnO is supposed to be single charge oxygen vacancies (V•O), and they suggest a mechanism where V•O are transformed to V•• O by tunneling of a hole trapped at the surface. A changed surface morphology could then easily change the prerequisite for this hole transfer, which is in line with the decrease in fluorescence observed. In the experiment, the sweep rate is slow enough (1 mV/s) for the surface to be in an equilibrium configuration. This is also further supported by the fact that the process is completely reversible, as seen in Figure 6b. The method also has the potential to induce population of deeper acceptor states. This would quench the fluorescence for applied potentials more negative than the level of these acceptor states. Experimentally, this does not happen, and the fluorescence at applied potentials more positive than 0.1 V vs NHE is the same as for a free-standing electrode not connected to the potentiostat. Since no decrease of the fluorescence is seen for applied potentials more negative than the acceptor trap states, there is no effective potentiostatic filling of the acceptor levels. This indicates either a lack of significant amounts of conductive acceptor states accessible by the potentistat, or alternatively a transport rate to the lower lying acceptor states slower than the radiative recombination (ktrans2 < krad), preventing them from being populated by the potentiostat under steady-state UV-illumination. It should be pointed out that it is not necessary for the entire fluorescence to have a

Figure 7. A sketch of the energy levels involved in the visible fluorescence. The absolute positions of the relevant energy levels are marked.

These measurements have been performed on a series of samples with particles varying from 5 to 8 nm in diameter. Smaller particles have a band gap higher than the excitation energy of the UV-led, and larger particles have passed the domain of quantum confinement. For ZnO particles around 4 nm in diameter and smaller, the trap states can move6 with respect to the conduction band which introduce further complications. For the particle size regime studied here, we see that the position of the fluorescence peak follows the position of the conduction band edge, meaning that the fluorescence peak occurs at less negative potentials for larger particles. The distance between the conduction band edge and the trap levels for ZnO quantum dots is thus constant and around 0.35 ± 0.03 eV regardless of band gap and particle size. A summary of the key data is found in Table 1, and more information can be found in the Supporting Information. Table 1. Summary of Key Data for the Analyzed Samples

5502

sample

Eg (eV)

diameter (nm)

band edge (nm)

peak position (V vs NHE)

distance to Ecb (eV)

1a 2a 3a 4a 5a 6a 7a

3.53 3.50 3.47 3.44 3.41 3.41 3.40

4.8 5.2 5.7 6.6 7.4 7.5 7.8

351 354 357 361 363 364 365

−0.63 −0.55 −0.61 −0.57 −0.51 −0.54 −0.54

0.36 0.41 0.32 0.32 0.36 0.33 0.32

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5. CONCLUSIONS In conclusion, we have demonstrated how the absolute energetic position of the trap levels involved in fluorescence can be determined by measuring fluorescence as a function of applied potential. The kinetic conditions for this method are quantified, and it should with relative ease be applicable to any nanoparticle or nanoporous semiconductor material showing fluorescence. In this paper, the method is exemplified by demonstrating its use for ZnO quantum dots of different sizes. The visible fluorescence in ZnO quantum dots is shown to occur by relaxation of electrons located at trap levels centered at 0.35 ± 0.03 eV below the conduction band edge down to acceptor states 2.65 eV below the conduction band edge. The measurements support that this fluorescence is connected to surface states. It is further demonstrated that the energetic distance between the position of the maximum of density of states for the upper trap levels and the conduction band edge is rather fixed at 0.35 ± 0.03 eV regardless of the particle size in the quantum confined regime. When ZnO particles increase in size, the conduction band edge is shifted to lower energies and the upper trap levels shift together with the conduction band.

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ASSOCIATED CONTENT

* Supporting Information S

Spectrum for the UV-led. Photocurrent during the potential sweep. Absorption data. Determination of the band gap and calculation of particle sizes. Comparison between the maximum of fluorescence as a function of potential with the optical band gap, the absorption coefficient, and differential absorption. Maximum fluorescence as a function of applied potential for all analyzed samples. Fluorescence peak position as a function of particle size and a comparison with the position of the conduction band edge. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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REFERENCES

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