Alan Snelson
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Photoconductivity in an Argon Matrix Containing Sodium and Tetracyanoethylene Alan Snelson IITResearch Institute, Chicago, lllinois 60676 (Received April 30, 7973)
A matrix-isolation cryostat was modified to allow photoconductivity measurements to be made. Argon matrices were deposited containing alternate layers of trapped sodium atoms and tetracyanoethylene molecules. These matrices on illumination exhibited photoconductivity. The photoconductivity mechanism is believed to be associated with the formation of donor-acceptor complexes between the trapped sodium and tetracyanoethylene,
Introduction The matrix isolation technique is now a well-established method for obtaining spectroscopic data for atomic, molecular, and radical species. Recently the method has been e ~ t e n d e d l -so ~ that the spectra of trapped ionic species may also be obtained. In this latter technique, suitable electron-donating and electron-accepting species are isolated within the same inert gas matrix and electron transfer between the donors and acceptors is promoted by photoexcitation.lS2 Electron spin resonance techniques have been used to characterize the isolated anionic and cationic ~ p e c i e s l -and ~ infrared spectra of a variety of matrix-isolated negative ions have been reported.4J~ The most commonly used electron donors have been alkali metal atoms. A variety of electron-accepting species have been used, N02,5 NO,5 S02,4 B2H6,3tetracyanoethylene,3 and furan.3 To date there does not appear to have been any attempt to detect photoconductivity in matrices containing electron-donating and -accepting species and it was the purpose of this study to determine if this phenomenon could be observed. Sodium (IP = 5.18 eV) and tetracyanoethylene (EA = 3.25 eV) were selected as the electrondonating and -accepting species, respectively. Experimental Section The basic matrix-isolation cryostat and molecular beam furnace used in this study has been described previously6 and only those details pertinent to the present experiments will be described. Liquid helium was used as the refrigerant. Matheson Research Grade argon was used as the matrix gas and was deposited a t the rate of 1.5 x 10-2 mol/hr-1. The infrared transmitting window on which matrices were normally deposited was replaced by a piece of glass with an electrical conducting surface (a deposit of tin oxide). This conducting surface formed one electrode of the photoconductor and matrices were deposited on this surface. The surface area of the window available for matrix deposition was rectangular, 0.75 in. X 0.5 in. A second rectangular electrode, 0.4 in. X 0.4 in., made of stainless steel 0.010 in. thick could be positioned directly in front of, and parallel to, the matrix on the conducting window after its deposition. This electrode was mounted in an insulated high-vacuum feed-through and its distance from the matrix surface could be varied from 0 to 1.5 in. Two Knudsen cells were constructed of steel to contain the sodium and TCNE (tetracyanoethylene). Both cells had orifices of 0.025 in. diameter and were mounted on separate arms on a shaft which could be rotated via a vacThe Journal of Physical Chemistry. Voi. 77, No. 20, 1973
uum feedthrough, so that one Knudsen cell a t a time could be aligned with the window on which the matrix was deposited. The Knudsen cells were heated electrically and their temperatures measured with chromel-alumel thermocouples. The orifices of both Knudsen cells when aligned with the cooled window were about 2.5 in. from the surface on which the matrices were formed. Sodium was vaporized at about 320" and TCNE at 70". These temperatures correspond to vaporization rates of 1.9 x mol hr-l for sodium7 and 3.7 X 10-4 mol hr-1 for TCNE.8 From these vaporization rates and the geometry of the system, the matrix dilution factors may be estimated at about 1300:l for the sodium and 670:l for the TCNE. Matrices were formed over a period of 10 min with alternate layers of sodium and TCNE being isolated. Each individual layer during the matrix formation was deposited over equal time periods, always starting with sodium and ending with TCNE. In various experiments deposition times for the individual layers of 10, 20, and 30 sec were used. At the end of the matrix deposition period, the liquid helium cold finger was rotated to a position a t which the matrix could be illuminated through the window on which it was deposited. A commercial 150-W tungsten filament light bulb was used for this purpose. The movable electrode was positioned a suitable distance from the matrix surface, separations of 0.25, 0.5, and 0.75 in. being typically used. A variable dc potential of from 0 to 2000 V was applied across the electrodes, and photocurrents were measured on a Keithley 601A electrometer. The latter instrument had a range of 10-3-10-11 A. Results The first series of experiments performed with the equipment were made to determine if any photoconductivity could be detected under the following conditions of illumination: (a) the matrix window alone a t 4.2"K, (b) the matrix window covered with argon at 4.2"K, (c) the matrix window covered with argon containing isolated sodium atoms at a dilution of approximately 1600:1, (d) the matrix window covered with argon containing isolated TCNE molecules at a dilution of approximately 670:1, (e) the matrix window covered with a thin layer of sodium, (f) the matrix window covered with a thin layer of TCNE, (9) the matrix window covered with alternate layers of pure sodium and TCNE. Applied voltages of up to 2000 V dc were tried, but no detectable photocurrents were observed.
2435
Photoconductivity in an Argon Matrix Containing Na and TCNE
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Figure 2. Photoconductivity curves for sodium and tetracyanoethylene in an argon matrix, 15 layers of each.
Figure 1. Photoconductivity curves for sodium and tetracyanoethylene in an argon matrix, 30 layers of each.
J
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TABLE I: Photocurrent with 3 0 Layers Each of Matrix-Isolated Sodium and TCNE Current, Aa
V
Ab
0
10 50 100 150 200 250 300 400 500 1000 1500 2000
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TABLE II: Photocurrent with 15 Layers Each of Matrix-Isolated Sodium and TCNE
e 5.1 X 7.4 x 10-9 8.6 X 1.8 X lo-* 2.6X 3.1 X
a In these measurements, the movable electrode was held in a fixed position and the voltage varied. No measurements were made in which the voltage was held constant and the electrode repositioned. Electrode 0.25 in. from matrix surface (see footnote a). Electrode 0.5 in. from matrix surface. Electrode 0.75 in. from matrix surface. e No measurements made.
Several experiments were then tried in which alternate matrix isolated layers of sodium and TCNE were deposited and attempts were made to detect photocurrents under illumination. The results from two typical “well-behaved” experiments are given in Table I and I1 and are presented graphically in Figures 1 and 2. Photoconductivity measurements made in the well-behaved experiments had the following characteristics. The photocurrent values were reproducible, stable, and did not show hysteresis effects. In order to obtain the photoconductivity it was necessary that the potential be applied so that the electrode adjacent to the first sodium layer was negative, and the movable electrode nearest the last TCNE layer was positive. If this polarity was reversed, no photocurrents were observed.
C
Figure 3. Schematic of photoconductivity process: (a) matrixisolated layers of sodium and TCNE without illumination; (b) matrix-isolated layers of sodium and TCNE ions formed on illumination; (c) photoconductivity due to charge transfer under t h e influence of an applied electric field.
Current. A V
Aa
Bb
0 10 50 100 200 300 400 500 1000 1500 2000
0 0 0
0
0
0
0 2.2 x 10-10 8.6 X 2.1 x 10-9 3.1x 10-9 3.7 x 10-9 4.3 x 10-9 6.3 x 10-9 8.0 x 10-9 8.7x 10-9
8.6 X 6.0 X 10-lo 1.1 x 10-9 1.8x 10-9 2.3 x 10-9 5.5 x 10-9 1.0 x 10-8 1.2 x 10-8
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a Electrode 0.25 in. from matrix surface. Electrode 0.5 in. from matrix surface. Electrode 0.75 in. from matrix surface.
In a few experiments matrices containing sodium and TCNE were prepared in which the resulting photocurrents were not very stable or reproducible. When this was the case relatively high photocurrents were recorded, some in the 10-5-10-4 range. In some cases a greenish glow appeared to emanate from the matrix and the photocurrent persisted for several seconds after the illumination was removed. It was also observed that the photocurrent occurred irrespective of the electrode polarity, though the magnitudes for a given potential difference were not equal. The Journal of Physical Chemistry, Vol. 77, No. 20, 1973
Alan Snelson
2436
Discussion On the basis of the experimental evidence there can be little doubt that photocurrents can be obtained in inert gas matrices containing layers of isolated sodium atoms and TCNE molecules. The source of this effect will be considered. Spectral response curves for the photocurrent were not obtained. However, the spectral response curve for the light source was crudely measured. Its output a t the shortest wavelength started at about 3700 A, with the bulk of the radiation being emitted between 5300 and 7000 A. The generation of photocurrents must therefore be justified in terms of this incident photon energy. The possibility that the photocurrents arise from the simple photoionization of solid argon, trapped sodium atoms, or trapped TCNE molecules is negated by the preliminary experimental data and also by the known magnitudes of the photoionization potentials for areon ( ~ 1 eV 3 < 950 A),9 sodium (5.8 eV < 2380 A),1 and TCNE (EA = 3.25 eV,I I P 5000 A or =3.20 eV. This latter quantity is markedly lower than the ionization potential of sodium atoms (5.8 eV). The light source used in this study would have sufficient energy, therefore, to cause the formation of matrix-isolated ion pairs. With the production of ion pairs, photoconductivity can be visualized as shown schematically in Figure 3. The experimental observation that photoconductivity could only be observed in the wellbehaved experiments when the electrode adjacent to the first sodium layer was negative supports the proposed conductivity mechanism. Photoconductivity data presented in Tables I and II indicate that somewhat larger photo-
The Journal of Physical Chemistry, Vol. 77, No. 20, 1973
currents were measured in the matrix containing a total of 30 layers of sodium and TCNE than in the matrix containing 15 layers. Qualitatively, this behavior would be expected, since more ion pairs and hence current carrying centers are formed in the former than in the latter matrix. The increase in photocurrent with applied voltage probably reflects the effect of the increasing electric field reducing the effective number of electron traps within the matrix. In this connection it has been shown in solid argon that oxygen impurities markedly affect electron mobilities and lifetimes.9 Obviously, doping the matrix in the present study with an excess of a strong electron acceptor, TCNE, is liable to produce many electron-trapping sites. A curious aspect of the photocurrent us. applied voltage curves is the apparently larger currents that were often obtained when one of the electrodes was farther from the matrix surface than closer to it. There does not seem to be any simple explanation for this behavior. Similarly, the production of large currents and greenish light emanations in the non-well-behaved experiments is not open to obvious interpretation,
Summary This preliminary study has clearly shown that photocurrents can be obtained from matrices containing isolated sodium atoms and TCNE molecules. The photoconduction mechanism almost certainly involves electron injection to an electron donor-acceptor complex formed by the trapped species. The system clearly requires further study, since some of the experimental observations are difficult to explain. From a practical point of view, it is possible that a matrix isolated donor-acceptor complex could be used as a memory device since it has been shown that once the complex is formed it is quite stable.1 A second interesting aspect of the device might be in the area of detection of electron-accepting and -donating materials in special cases. From data obtained in this study for the well-behaved matrix it may be estimated that the presence of at least 1012 donor-acceptor complexes can be detected. With a more thorough understanding of the photoconduction process, this already high sensitivity could possibly be increased. References and Notes (1) P. H. Kasia, Phys. Rev. Lett., 21, 67 (1968). (2) P. H. Kasai and D.McLeod, Jr., J. Chem. Phys., 51, 1250 (1969) (3) P. H. Kasai. Accounts Chem. Res.. 4.329 (1971). i 4 j D. E. Milligan and M. E. Jacox, J. Chem. Phys., 55, 1003 (1971). (5) D.E. Milligan and M. E. Jacox, J. Chem. Phys., 55, 3404 (1971). (6) A. Snelson, J . Phys. Chem., 73, 1919 (1969). (7) W. T. Hicks, J. Chem. Phys., 38, 1873 (1963). (8) R. H. Boyd, J. Chem. Phys., 38, 2529 (1963). (9) L. S. Miller, S. Howe, and W. E. Spear, Phys. Rev., 166, 871 (1968).
