PbS Nanodots Embedded in ZrO2 Thin Films for Ultraviolet Radiation Dosimetry

Yu. Dekhtyar1, M. Romanova1, A. Anischenko1, A. Sudnikovich1, N. Polyaka1, R. Reisfeld2, T. Saraidarov2 and B. Polyakov3

1 Institute of Biological Engineering and Nanotechnology, Riga Technical University, Riga, Latvia

2 Hebrew University of Jerusalem, Jerusalem, Israel 3 Institute of Solid State Physics, University of Latvia, Riga, Latvia

Abstract— PbS nanodots embedded in ZrO2 thin film matrix (ZrO2:PbS films) were investigated for UV radiation dosimetry purposes. ZrO2:PbS films were UV irradiated using wavelengths 250 – 400 nm. Photoelectron emission spectra of ZrO2:PbS films were recorded and band structure of the films was calculated. It was found that density of localized states increased with increase in concentration of PbS nanodots which allowed to suggest that PbS nanodots are responsible for creation of localized states. Number of localized states decreased after UV irradiation. The linear correlation between number of localized states and time of UV exposure was observed. Observed changes in band structure of ZrO2:PbS films under the influence of UV irradiation suggest that the films may be considered as an effective material for UV radiation dosimetry, PbS nanodots being the UV sensitive substance of the films.

Keywords— PbS nanodots, ultraviolet radiation, dosimetry, photoelectron emission.

I. INTRODUCTION

Damage of biological structures caused by ultraviolet (UV) radiation depends on interaction of UV photons with either DNA or other biomolecular structures. These structures are scaled to nanodimensions, therefore, it is necessary to have an UV sensor of corresponding nano volume. This experimental work offers to use a thin film dosimeter, which consists of nanodots embedded in a solid thin film matrix. The nanodots are supposed to be a radiation-sensitive substance. The signal from the dosimeter can be detected by measuring emission of low energy photo excited electrons (~ 1 eV) which have a mean free path of the order of several nanometers.

The target of the research was to examine changes in photoemission properties and band structure of the samples with lead sulphide (PbS) nanodots embedded in zirconia (ZrO2) thin film matrix under the influence of UV radiation. PbS nanodots were chosen for their possible application as an UV radiation dosimeter because it has been reported that they have emission and absorption lines in a large spectral region [1]. To eliminate the influence of ZrO2 matrix and prove that the nanodots not the matrix are sensitive to radiation, samples with PVA matrix were studied as well.

II. SAMPLES

Both ZrO2:PbS and PVA:PbS films were fabricated using sol-gel technique [1]. Samples with 10%, 20% and 50% concentration of PbS in ZrO2 matrix and 20% PbS in PVA matrix were studied. All films were deposited on a glass substrate. Thickness of the films was in a range of 0.1-1 m. Typical size of PbS nanodots was 2-4 nm in ZrO2 matrix and 2-3 nm in PVA matrix. To verify the size of the nanodots, the atomic force microscope Solver P-47 PRO was employed.

III. METHODS

The samples were UV irradiated using HAMAMATSU PHOTONICS xenon-mercury lamp L8222 (250 – 400 nm) during 0-50 min. The photoemission (PE) current of irradiated and nonirradiated samples was recorded; a handmade spectrometer was used (inaccuracy of quantum energy measurement ±0.03 eV in wavelength range 200 – 300 nm). The photoemission was excited by photons in energy range 4-6 eV provided by the deuterium lamp source (LOT-Oriel Europe). Emitted electrons were detected using the secondary electron multiplier (VEU-6, Russia) in vacuum condition 105 torr.

IV. RESULTS AND DISCUSSION

Derivatives of PE current for nonirradiated ZrO2:PbS and PVA:PbS films are shown in Figure 1.

ZrO2:50%PbS film has an emission interval 4.8 – 6 eV with a maximum at 5.5 eV. The interval width is 1.2 eV, which is broad enough to be associated with the emission of electrons from localized states located inside the energy gap. A monotonous rise of dI/dhv curve when photon energy exceeds 6 eV might belong to the edge region of the valence band.

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Fig. 1. Derivatives of PE current of nonirradiated ZrO2:PbS films: (1) ZrO2:10%PbS; (2) ZrO2:20%PbS; (3) ZrO2:50%PbS; (4) PVA:20%PbS.

Derivatives of PE current of ZrO2 films with 10% and 20% concentration of PbS have an inflection point at the same photon energy as the maximum detected for ZrO2:50%PbS films (5.5 eV). The inflection point is followed by a monotonous rise of dI/dhv curve when photon energy exceeds 5.7 eV which is similar to the behavior of ZrO2:50%PbS spectrum over 6 eV. Lack of well-defined emission maximum allows to suppose that localized states and the valence band are overlapped for ZrO2:10%PbS and ZrO2:20%PbS films, and concentration of localized states is not sufficient to provide an emission maximum.

The shape of PVA:20%PbS derivative is similar to ZrO2:50%PbS derivative but the maximum is shifted to 5.7 eV and the curve starts to rise over 6.35 eV. However, the shape of PVA:20%PbS derivative differs from that of ZrO2:20%PbS derivative. Taking into account the similarities in the shapes of the derivatives, it was possible to suppose that localized states were created by PbS nanodots but PVA matrix had strong influence on position of localized states.

Fig. 2. Photoemission spectra of UV irradiated ZrO2:PbS films: (1) ZrO2:10%PbS; (2) ZrO2:20%PbS; (3) ZrO2:50%PbS (4) PVA:20%PbS.

Figure 2 shows derivatives of the PE current for the films irradiated with UV for 50 minutes. The emission maximums shown in Figure 1 disappear after UV irradiation. That allows to suppose that number of localized states decreases during UV irradiation.

Band structure of the films was calculated and its changes under influence of UV irradiation were analyzed in accordance with the scheme shown in Figure 3. All band structure parameters were calculated using the spectra shown in Figures 1 and 2. The results are presented in Figure 4.

Fig. 3. The scheme for band structure calculation of ZrO2:PbS and PVA:PbS films: – electron work function; Eg – energy gap; W – energy

needed to release an electron from the valence band; – half width of localized states; E1 – distance between the valence band and the midpoint of localized states; – electron affinity. W and were calculated from the

recorded photoemission current. To calculate W, the tail of a spectrum which is related to the valence band was approximated using MS Excel and

further extrapolated to I=0. =W-Eg; E1=W-( + )

Figure 4 shows: 1. Number of localized states decreases under the

influence of UV radiation for both ZrO2:PbS and PVA:PbS films. Tails appear at the edge of the valence band for nonirradiated ZrO2:10%PbS and ZrO2:20%PbS films. Localized states of ZrO2:50%PbS film disappear completely after 50 minutes of UV irradiation.

2. The position E1 of localized states has a trend to be closer to the edge of the valence band when concentration of PbS increases.

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Fig. 4. Band structure of ZrO2:PbS and PVA:PbS films. The horizontal axis shows density of states, the vertical axis shows energy [eV]

3. The electron affinity increases with increase in PbS concentration in ZrO2 matrix (Fig. 5). According to [2], increase in PbS concentration results in formation of larger PbS nanodots. Therefore, it is possible that larger nanodots are characterized with higher values.

Fig. 5. Electron affinity of ZrO2:PbS films as a function of PbS concentration: (1) nonirradiated films; (2) irradiated films (50 minutes)

The electron affinity of the irradiated ZrO2:PbS films is smaller than that of the nonirradiated films (Fig. 5). However, the decrease in affinity after irradiation depends on PbS concentration (Fig. 6). Value is smaller

in films with higher concentration of PbS. At the same time in PVA:20%PbS films (0.75 eV) is significantly higher

than in ZrO2:20%PbS films (0.20 eV), not shown in the Figure. It means that the matrix has strong influence on .

Fig. 6. Decrease in electron affinity after irradiation for ZrO2: PbS films

4. The half width of localized states increases with increase in PbS concentration in ZrO2:PbS films (Fig. 7). It might mean that density of localized states increases with increase in PbS concentration. It also might be one more evidence that PbS nanodots are responsible for creation of localized states.

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Fig. 7. The half width of localized states of ZrO2:PbS films as a function of PbS concentration: (1) nonirradiated films; (2) irradiated films (50

minutes)

Density of localized states of ZrO2:PbS films with different concentrations of the nanodots can be evaluated by calculating areas below the 5.5 eV PE maximums shown in Figures 1 and 2 (Fig. 8). Density of localized states increases with increase in PbS concentration, films with 50% PbS having the greatest value. UV decreases the area which might mean that UV knocks electrons out of localized states that is in accordance with the results from Figure 4.

Fig. 8. The area below the photoemission maximum (5.5 eV) of ZrO2:PbS films as a function of PbS concentration: (1) nonirradiated films; (2)

irradiated films (50 minutes)

Fig. 9. Dependence of the area below the 5.5 eV photoemission maximum

of ZrO2:50%PbS and PVA:20%PbS (secondary axis) films on UV exposure

Number of localized states depends on irradiation time (Fig. 9). ZrO2:PbS films have linear correlation between number of states (represented by the area) and time of UV exposure (representing different doses of UV radiation). Therefore, ZrO2:PbS films are considered to be an effective material for UV dosimetry purposes. The linear correlation isn’t observed for PVA:PbS films.

Photoemission current of ZrO2 and PVA matrixes was recorded as well (not shown in the Figures). UV radiation did not change these spectra significantly. This gave more evidence that the emission maximums shown in Figure 1 were provided by PbS nanodots, meaning that PbS nanodots not the matrix is the UV sensitive substance of the films.

V. CONCLUSIONS

1. Density of localized states of ZrO2:PbS films increases with PbS concentration.

2. The matrix of films influences density of localized states. Density of localized states is higher in ZrO2 matrix than in PVA matrix.

3. There is a linear correlation between UV radiation exposure and number of localized states for ZrO2:PbS films.

4. The half width of localized states in ZrO2:PbS films increases with PbS concentration that might depend on increase in nanodots size. Increase of might evidence that localized states are created by PbS nanodots.

5. The electron affinity in ZrO2:PbS films decreases after UV irradiation and increases with increase in PbS concentration. Higher concentrations of PbS nanodots result in smaller decrease after UV irradiation.

6. ZrO2:PbS films may be considered as a suitable material for UV radiation dosimetry, PbS nanodots being the UV sensitive substance of such a dosimeter.

REFERENCES

1. Sashchiuk A, Lifshitz E, Reisfeld R et al. (2002) Optical and conductivity properties of PbS nanocrystals in amorphous zirconia sol-gel films. J Sol-Gel Sci Technol 24:31–38.

2. Saraidarov T, Reisfeld R, Sashchiuk A, Lifshitz E. (2003) Synthesis and characterization of lead sulfide nanoparticles in zirconia-silica-urethane thin films prepared by sol-gel method. J Sol-Gel Sci Technol 26:533–540.

Author: Marina Romanova Institute: Riga Technical University Street: Ezermalas 6b, 231 City: Riga Country: Latvia Email: marina.romanova@inbox.lv

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