- Email: [email protected]
Blue light-emission from a nanostructured organic polymer semiconductor
Solid State Communications, Vol. 95, No. 3, p. 185-189, 1995 Copyright 0 1995 Ee, vier Science Ltd Printed% &ea~Britain. All ri@ts reserved 0038-1098/95 $9.50 + .@I
0038-1098(95)00193-x
BLUE LIGHT-EMISSION
FROM A NANOSTRUCTURED SEMICONDUCTOR
ORGANIC POLYMER
D.B. Romero, M. Schaer, J.L. Staehli and L. Zuppiroli Department
of Physics, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland and G. Widawski, M. Rawiso, B. Francois Institut Charles Sadron, 67083 Strasbourg, France (Received 21 November 1994; accepted 7 March 1995 by P. Wachter)
We report on the enhancement of the photoluminescence (PL) intensity from a nanostructured organic material composed of nanosized aggregates of poly(para-phenylene) (PPP) in a polystyrene insulating matrix. In addition, the peak of the PL emission spectrum from these nanostructured organic semiconductors is blue-shifted from that of unstructured PPP. Furthermore, these PL characteristics correlate with the presence of an unusual sub-gap absorption feature that is present only in the nanostructured PPP. These results suggest the departure of the optical properties of random nanostructured organic polymers from their unstructured counterparts. Keywords: A. nanostructures, A. polymers, elastomers, and plastics, D. optical properties, E. neutron scattering.
RESEARCH ON inorganic semiconductors has shown that by fabricating structures with reduced dimensions such as quantum wells and superlattices [l] and more random nanostructures [2] like porous silicon, one can control the color and increase the efficiency of light emission from these systems. These effects are consequences of the modifications of the bulk optical properties induced by changes in the electronic states as these materials are reduced to sizes comparable to the de-Broglie wavelength of the charge carriers of the system. In view of recent interest in the possible use of organic materials as lightemitting devices [3-51, it is interesting to speculate if one can take advantage of the quantum-size effects mentioned above to improve the luminescence properties of organic polymer semiconductors. In this communication, we attempt to address this issue by presenting evidence of the amelioration of the emission characteristics of an organic semiconductor upon formation of a random nanostructured system. The nanostructured organic semiconductor in this work is based on the organic polymer poly(paraphenylene) (PPP). To digress briefly, we note that a
primary requirement in fabricating organic polymer light-emitting devices is that the organic polymer must be soluble to allow its deposition onto a substrate by spin-coating. However, pure PPP is an intractable and insoluble material. In order to address this problem, two of us (G.W. and B.F.) have synthesized [6] a polystyrene-poly(para-phenylene) (PS-PPP) block copolymer in which polystyrene (PS) conferred the desired solubility to the system. Besides improving the solubility, PS-PPP offers another advantage. In solution, the insoluble PPP blocks could assemble into nanosized aggregates. [From hereon, we will refer to these nanosized PPP aggregates as nanostructured PPP (n-PPP).] We point out that this ability to selforganize in solution has been demonstrated in other copolymer systems containing soluble-insoluble saturated sequences [7]. However, an important new feature of PS-PPP is that the PPP block is an electroactive conjugated polymer. The present study is concerned with the possible modifications in the electronic states of these conjugated nanostructured PPP induced by quantum confinement. The first part of the paper presents evidence from small angle 185
186
BLUE LIGHT-EMISSION
Vol. 95, No. 3
FROM A SEMICONDUCTOR 0.6
l
o
‘P
n
0 0
0
_ 0.10
WA’)
0.15
Fig. 1. Typical PPP partial scattering functions for different copolymer concentration.
Q(A’) Fig. 2. Typical PPP form factors for different copolymer concentrations.
neutron scattering experiments on PS-PPP in solution pertinent to the formation of the nanostructured PPP. From these results, we will extrapolate the persistence of n-PPP to the solid state (i.e., in our thin films). The second part of the paper deals with the optical properties of n-PPP. We will demonstrate that they depart from those of pure PPP. In particular, we will report on the increase by more than an order of magnitude of the photoluminescence (PL) intensity and on the blue-shift of the PL emission spectrum from n-PPP. Furthermore, we will also show the correlation between these emission characteristics and the presence of an unusual sub-gap absorption feature in n-PPP. At the end, possible interpretations of these results will be discussed. The neutron scattering experiments were performed on PS-PPP diblock copolymer in carbon disulphide solutions of various concentrations approaching the solid state. We note that since the coherent scattering length of the carbon disulphide solvent is close to that of the PS sequence, only scattering from the PPP part would be observable from the solutions. In Fig. 1, we show the typical variation of the PPP partial scattering intensity as a function of the magnitude of the scattering wavevector (Q) for different copolymer concentration. In general, the scattering functions depicted in Fig. 1 are convolutions of the structure factor, G(Q), and the form factor, S(Q). Qualitatively, G(Q) would reveal information on the structures which are formed by the block copolymer system in solution whereas S(Q) contains information about the possible correlations between these structures. Such information extracted from the data presented in Fig. 1 is discussed below. At high Q, the tail of the scattering functions of Fig. 1 corresponds closely to G(Q) since S(Q) approaches unity in this limit. Thus, after a suitable
correction of the incoherent part of each scattering spectrum, these data are re-plotted as Q4G(Q) vs Q in Fig. 2. The curves in FiF. 2 saturate to nearly the same value for Q > O.O6A- for various concentrations of PS-PPP in solution. Therefore, G(Q) decreases as Q-4 at high Q values, a behavior that suggests the existence of a sharp interface between the PPP and PS parts [8]. From this result, one can infer that the PPP parts aggregate in solution. The size of this PPP aggregate is related to the saturation value [Q4G(q)lsat, of the curves shown in Fig. 2. As a first approximation, the surface-to-volume ratio of the PPP aggregate is given by [Q4G(Q)lsat 2 27r(S/V) [8]. Assuming a spherical shape for the nanostructured PPP, one could then calculate their average size from [Q4G(Q)]$,,. For the samples used in the present work, the sizes are in the range 4- 10 nm. Moreover, in addition to being able to infer the size of the aggregate, the near overlap of the data for different PS-PPP concentration as depicted in Fig. 2 is a proof that the size of the n-PPP is well defined and independent of the copolymer concentration. Consequently, we are able to conclude from these results that the n-PPP persists in thin fihns fabricated from solutions. A peak in the PPP scattering spectrum is prominent in the data of Fig. 1 at low Q values. This peak sharpens and its position (Q,,,) shifts towards higher Q values as the solid state condition is approached. In real space, Qmax corresponds to the mean distance (d& between the nanosized PPP aggregates which is given by the simple relation da,, = 27rlQmax. This quantity versus the copolymer concentration (c), from a 2% solution up to the solid state, is presented in Fig. 3. The subsequent curve is close to the d cc c- ‘I3 dependence which corresponds to a near perfect dispersion of n-PPP in the PS matrix. This corroborates further the conclusion
Vol. 95, No. 3
I
0.01
BLUE LIGHT-EMISSION
.
. .
A....’
.
FROM A SEMICONDUCTOR
187
.‘....I
0.1 Concentration (gkm3)
1
Fig. 3. Average distance (d,,,) between PPP cores vs copolymer concentration. that well-defined PPP aggregates are formed from PS-PPP in solution. We now turn our attention to the results of our optical investigations. The samples used in our optical studies are l-2 pm thick films of PS-PPP prepared from solutions in chloroform containing 4% PS-PPP by weight. Transmission measurements were conducted on free-standing films using standard commercially available grating spectrometers covering a wide range of frequencies. The real part of the frequency dependent complex conductivity (ui) was obtained following the Kramers-Kronig procedure previously developed to analyse the transmission data of highT, superconductors [9]. Photoluminescence (PL) measurements were carried out on samples cast onto glass substrates using the unfiltered ultraviolet lines of an Argon-ion laser as excitation, defocused at the sample position to avoid thermally-induced effects. The incident laser power intensity was approximately 30mWcm-2. The light emitted from the sample was focused onto the entrance slit and dispersed inside a single grating monochromator. In order to discriminate the weak luminescence from the sample, standard lock-in detection technique was used. The output signal was modulated by chopping the exciting laser light. This modulated signal was detected with a photomultiplier. The photomultiplier output was then recorded with a lock-in amplifier tuned at the laser chopping fequency. Figure 4 compares the photoluminescence (PL) spectra at different stages of the thermal conversion in vacuum of n-PPP to pure PPP. Total conversion to pure PPP was achieved at a temperature of 450°C. The PL spectrum of n-PPP shows a strong peak at X N 420nm and a weaker shoulder-like feature around X 2 500nm. For films which underwent the
600
600
400
3
1 (nm)
Fig. 4. Photoluminescence spectra at various stages of thermal conversion in vacuum from the nanostructured PPP (n-PPP) to pure PPP. The numbers in parentheses are scaling factors used to aid in the comparison of the features of each spectrum. thermal treatment procedure, one observes an overall diminution of the PL intensity and in particular the one near X N 420 nm. With pure PPP, only a single peak around X N 480 nm can be discerned. We also note that this spectrum is in good agreement with the one reported by Grem et al. [5] of pure PPP obtained by a different synthetic route. The data in Fig. 4 reveal an increase of the PL intensity (nearly a factor of 40 in this case) from n-PPP as compared to that of pure PPP. Furthermore, the emission peak of the n-PPP spectrum is blue-shifted from that of pure PPP. This result implies that different electronic states are involved in light emission from nanostructured and pure PPP. Further insight into the origin of the unusual PL characteristics of the nanostructured PPP can be derived from an examination of the electronic transitions near the band-edge as demonstrated in Fig. 5. The al-curve of pure PPP has a single peak near X N 340 nm. Previous studies [lo] assigned this peak to the PPP 7r--n*-band transition energy. In contrast, the n-PPP spectrum shows three peaks. The highest energy peak (X N 270nm) could be attributed to absorption by the polystyrene (PS) matrix due to its proximity to a prominent PS spectral feature and its disappearance in the spectrum of the fully converted PPP film. The peak near X N 320 nm could be identified as a blue-shifted PPP r-n* band transition energy. This assignment is based on earlier works [3, lo] which showed such blue-shift of the main optical absorption feature as the effective conjugation length of the polymer was reduced. It is the presence of the sub-gap feature around X N 420 nm
BLUE LIGHT-EMISSION
188
FROM A SEMICONDUCTOR
50
600
400
600
0 200
h(nm)
Fig. 5. Comparison of the photoluminescence and optical conductivity of nanostructured PPP (n-PPP) and pure PPP. that is unusual. The previous studies [3, lo] just mentioned did not reveal such sub-gap features. We suggest that the unique “quantum-dot” like structure of n-PPP give rise to this sub-gap absorption feature. In Fig. 5, we superimposed the PL spectra relative to the features in the absorption data of both the nanostructured and pure PPP samples. For pure PPP (dash-dot curves), the PL peak is Stokes-shifted from the main absorption peak. This behavior, typical of organic polymer semiconductors, has been attributed [I l] to the relaxation of the photo-excited electronhole quasiparticles to polaronic states within the (x-r*)-gap prior to radiative recombination. On the other hand, for n-PPP, the position of the PL peak coincides with the sub-(n-x*)-gap absorption feature. We infer from this result that direct radiative transition from the electronic sub-gap states induced by quantum confinement is responsible for the PL features of n-PPP. What we have just demonstrated is that n-PPP possesses optical characteristics which deviate from those of unstructured PPP. Below, we discuss two possible mechanisms which could account for these observed deviations. The first mechanism concerns states at the interface of n-PPP and polystyrene. Due to the significant increase of the surface-to-volume ratio in nanostructured systems, photon absorption due to optical transitions involving the interface states can be comparable to those of the bulk-states. Such optical transitions would account for the sub-gap spectral feature observed in n-PPP. Also, the localized nature of these interface states may explain the enhancement of the PL efficiency of the PPP quantum dots.
Vol. 95, No. 3
The second mechanism is related to the possibility of creating bound polaronic-exciton states in nanostructured organic semiconductors. A qualitative argument for this scenario is as follows. Recall that, from the neutron scattering data, the typical size of the n-PPP is between 4 and 10nm. Knowing that the monomer length in the phenyl group is about 0.4 nm, then a fully extended chain within the n-PPP will contain lo-25 monomer units. Since the delocalization length for a polaron [12] is typically a few monomer units, there is sufficient space for the formation of these quasiparticles within n-PPP. Moreover, since the delocalization length for a polaron is comparable to the size of n-PPP, it is plausible to expect an enhancement of the overlap between the quasiparticle wavefunctions. Thus, Coulomb interaction between the polaronic charge carriers is increased. This condition favors the formation of bound polaronic-exciton states. The sub-gap feature in the absorption spectra of n-PPP is attributed to transitions to these new electronic states induced by the spatial confinement. Also, the enhanced quasiparticle wavefunction overlap could explain the increase in the probability for radiative transitions which would account for the enhancement of the PL intensity in n-PPP. To conclude, the nanostructured organic semiconductor presented in this work revealed interesting new optical properties. It manifested an enhancement of the intensity and a blue-shift in the color of its emission characteristics. These improved emission properties could come from radiative transitions from unusual sub-gap electronic states induced by quantum confinement. By direct comparison, we have shown that these features depart from the luminescence and absorption characteristics of its unstructured counterpart. These modifications in their optical properties have useful implications towards the utilization of nanostructured organic semiconductors in light-emitting devices. - D.B.R. and L.Z. acknowledge the support for this work from the Optics Priority Program in Switzerland. G.W., M.R. and B.F. gratefully acknowledge the material help and advice from F. BovC, L. Noirez, L. Auvray, A. Brulet and J.P. Cotton. Acknowledgements
REFERENCES 1. 2. 3.
See the article of G.F. Neumark, R.M. Park & J.M. DePuydt, Physics Today 47,26 (1994) (and references cited). For a review, A.D. Yoffe, Adv. Phys. 42, 173 (1993). P.L. Burn et al., Nature 356,47 (1992).
Vol. 95, No. 3 4. 5. 6.
7. 8.
9.
BLUE LIGHT-EMISSION
FROM A SEMICONDUCTOR
G. Gustafsson et al., Nature 357, 477 (1992). G. Grem et al., Adv. Mater. 4, 36 (1992). B. Fraqois & X.F. Zhong, Synth. Met. 41-43, 955 (1991); X.F. Zhong & B. Francois, Makromol. Chem. 192, 2277 (1991). G.A. McConnell et al., Phys. Rev. Lett. 71,2102 (1993). L. Auvray & P. Auroy, Neutron, X-Ray and Light Scattering (Edited by P. Linderer & Th. Zemb). Elsevier (1991). D.B. Romero et al., Solid State Commun. 82, 183 (1992).
10.
11. 12.
189
G. Leising, K. Pichler & F. Stelzer, in Electronic Properties of Conjugated Polymers ZZZ(Edited by H. Kuzmany, M. Mehring & S. Roth) Vol. 91, p. 100. Springer-Verlag, Berlin (1989). R.H. Friend, D.D.C. Bradley & P.D. Townsend, J. Phys. D: Appl. Phys. 20, 1367 (1987). A.J. Heeger, S. Kivelson, J.R. Schrieffer & W.P. Su, Rev. Mod. Phys. 60,781 (1988).
0038-1098(95)00193-x
BLUE LIGHT-EMISSION
FROM A NANOSTRUCTURED SEMICONDUCTOR
ORGANIC POLYMER
D.B. Romero, M. Schaer, J.L. Staehli and L. Zuppiroli Department
of Physics, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland and G. Widawski, M. Rawiso, B. Francois Institut Charles Sadron, 67083 Strasbourg, France (Received 21 November 1994; accepted 7 March 1995 by P. Wachter)
We report on the enhancement of the photoluminescence (PL) intensity from a nanostructured organic material composed of nanosized aggregates of poly(para-phenylene) (PPP) in a polystyrene insulating matrix. In addition, the peak of the PL emission spectrum from these nanostructured organic semiconductors is blue-shifted from that of unstructured PPP. Furthermore, these PL characteristics correlate with the presence of an unusual sub-gap absorption feature that is present only in the nanostructured PPP. These results suggest the departure of the optical properties of random nanostructured organic polymers from their unstructured counterparts. Keywords: A. nanostructures, A. polymers, elastomers, and plastics, D. optical properties, E. neutron scattering.
RESEARCH ON inorganic semiconductors has shown that by fabricating structures with reduced dimensions such as quantum wells and superlattices [l] and more random nanostructures [2] like porous silicon, one can control the color and increase the efficiency of light emission from these systems. These effects are consequences of the modifications of the bulk optical properties induced by changes in the electronic states as these materials are reduced to sizes comparable to the de-Broglie wavelength of the charge carriers of the system. In view of recent interest in the possible use of organic materials as lightemitting devices [3-51, it is interesting to speculate if one can take advantage of the quantum-size effects mentioned above to improve the luminescence properties of organic polymer semiconductors. In this communication, we attempt to address this issue by presenting evidence of the amelioration of the emission characteristics of an organic semiconductor upon formation of a random nanostructured system. The nanostructured organic semiconductor in this work is based on the organic polymer poly(paraphenylene) (PPP). To digress briefly, we note that a
primary requirement in fabricating organic polymer light-emitting devices is that the organic polymer must be soluble to allow its deposition onto a substrate by spin-coating. However, pure PPP is an intractable and insoluble material. In order to address this problem, two of us (G.W. and B.F.) have synthesized [6] a polystyrene-poly(para-phenylene) (PS-PPP) block copolymer in which polystyrene (PS) conferred the desired solubility to the system. Besides improving the solubility, PS-PPP offers another advantage. In solution, the insoluble PPP blocks could assemble into nanosized aggregates. [From hereon, we will refer to these nanosized PPP aggregates as nanostructured PPP (n-PPP).] We point out that this ability to selforganize in solution has been demonstrated in other copolymer systems containing soluble-insoluble saturated sequences [7]. However, an important new feature of PS-PPP is that the PPP block is an electroactive conjugated polymer. The present study is concerned with the possible modifications in the electronic states of these conjugated nanostructured PPP induced by quantum confinement. The first part of the paper presents evidence from small angle 185
186
BLUE LIGHT-EMISSION
Vol. 95, No. 3
FROM A SEMICONDUCTOR 0.6
l
o
‘P
n
0 0
0
_ 0.10
WA’)
0.15
Fig. 1. Typical PPP partial scattering functions for different copolymer concentration.
Q(A’) Fig. 2. Typical PPP form factors for different copolymer concentrations.
neutron scattering experiments on PS-PPP in solution pertinent to the formation of the nanostructured PPP. From these results, we will extrapolate the persistence of n-PPP to the solid state (i.e., in our thin films). The second part of the paper deals with the optical properties of n-PPP. We will demonstrate that they depart from those of pure PPP. In particular, we will report on the increase by more than an order of magnitude of the photoluminescence (PL) intensity and on the blue-shift of the PL emission spectrum from n-PPP. Furthermore, we will also show the correlation between these emission characteristics and the presence of an unusual sub-gap absorption feature in n-PPP. At the end, possible interpretations of these results will be discussed. The neutron scattering experiments were performed on PS-PPP diblock copolymer in carbon disulphide solutions of various concentrations approaching the solid state. We note that since the coherent scattering length of the carbon disulphide solvent is close to that of the PS sequence, only scattering from the PPP part would be observable from the solutions. In Fig. 1, we show the typical variation of the PPP partial scattering intensity as a function of the magnitude of the scattering wavevector (Q) for different copolymer concentration. In general, the scattering functions depicted in Fig. 1 are convolutions of the structure factor, G(Q), and the form factor, S(Q). Qualitatively, G(Q) would reveal information on the structures which are formed by the block copolymer system in solution whereas S(Q) contains information about the possible correlations between these structures. Such information extracted from the data presented in Fig. 1 is discussed below. At high Q, the tail of the scattering functions of Fig. 1 corresponds closely to G(Q) since S(Q) approaches unity in this limit. Thus, after a suitable
correction of the incoherent part of each scattering spectrum, these data are re-plotted as Q4G(Q) vs Q in Fig. 2. The curves in FiF. 2 saturate to nearly the same value for Q > O.O6A- for various concentrations of PS-PPP in solution. Therefore, G(Q) decreases as Q-4 at high Q values, a behavior that suggests the existence of a sharp interface between the PPP and PS parts [8]. From this result, one can infer that the PPP parts aggregate in solution. The size of this PPP aggregate is related to the saturation value [Q4G(q)lsat, of the curves shown in Fig. 2. As a first approximation, the surface-to-volume ratio of the PPP aggregate is given by [Q4G(Q)lsat 2 27r(S/V) [8]. Assuming a spherical shape for the nanostructured PPP, one could then calculate their average size from [Q4G(Q)]$,,. For the samples used in the present work, the sizes are in the range 4- 10 nm. Moreover, in addition to being able to infer the size of the aggregate, the near overlap of the data for different PS-PPP concentration as depicted in Fig. 2 is a proof that the size of the n-PPP is well defined and independent of the copolymer concentration. Consequently, we are able to conclude from these results that the n-PPP persists in thin fihns fabricated from solutions. A peak in the PPP scattering spectrum is prominent in the data of Fig. 1 at low Q values. This peak sharpens and its position (Q,,,) shifts towards higher Q values as the solid state condition is approached. In real space, Qmax corresponds to the mean distance (d& between the nanosized PPP aggregates which is given by the simple relation da,, = 27rlQmax. This quantity versus the copolymer concentration (c), from a 2% solution up to the solid state, is presented in Fig. 3. The subsequent curve is close to the d cc c- ‘I3 dependence which corresponds to a near perfect dispersion of n-PPP in the PS matrix. This corroborates further the conclusion
Vol. 95, No. 3
I
0.01
BLUE LIGHT-EMISSION
.
. .
A....’
.
FROM A SEMICONDUCTOR
187
.‘....I
0.1 Concentration (gkm3)
1
Fig. 3. Average distance (d,,,) between PPP cores vs copolymer concentration. that well-defined PPP aggregates are formed from PS-PPP in solution. We now turn our attention to the results of our optical investigations. The samples used in our optical studies are l-2 pm thick films of PS-PPP prepared from solutions in chloroform containing 4% PS-PPP by weight. Transmission measurements were conducted on free-standing films using standard commercially available grating spectrometers covering a wide range of frequencies. The real part of the frequency dependent complex conductivity (ui) was obtained following the Kramers-Kronig procedure previously developed to analyse the transmission data of highT, superconductors [9]. Photoluminescence (PL) measurements were carried out on samples cast onto glass substrates using the unfiltered ultraviolet lines of an Argon-ion laser as excitation, defocused at the sample position to avoid thermally-induced effects. The incident laser power intensity was approximately 30mWcm-2. The light emitted from the sample was focused onto the entrance slit and dispersed inside a single grating monochromator. In order to discriminate the weak luminescence from the sample, standard lock-in detection technique was used. The output signal was modulated by chopping the exciting laser light. This modulated signal was detected with a photomultiplier. The photomultiplier output was then recorded with a lock-in amplifier tuned at the laser chopping fequency. Figure 4 compares the photoluminescence (PL) spectra at different stages of the thermal conversion in vacuum of n-PPP to pure PPP. Total conversion to pure PPP was achieved at a temperature of 450°C. The PL spectrum of n-PPP shows a strong peak at X N 420nm and a weaker shoulder-like feature around X 2 500nm. For films which underwent the
600
600
400
3
1 (nm)
Fig. 4. Photoluminescence spectra at various stages of thermal conversion in vacuum from the nanostructured PPP (n-PPP) to pure PPP. The numbers in parentheses are scaling factors used to aid in the comparison of the features of each spectrum. thermal treatment procedure, one observes an overall diminution of the PL intensity and in particular the one near X N 420 nm. With pure PPP, only a single peak around X N 480 nm can be discerned. We also note that this spectrum is in good agreement with the one reported by Grem et al. [5] of pure PPP obtained by a different synthetic route. The data in Fig. 4 reveal an increase of the PL intensity (nearly a factor of 40 in this case) from n-PPP as compared to that of pure PPP. Furthermore, the emission peak of the n-PPP spectrum is blue-shifted from that of pure PPP. This result implies that different electronic states are involved in light emission from nanostructured and pure PPP. Further insight into the origin of the unusual PL characteristics of the nanostructured PPP can be derived from an examination of the electronic transitions near the band-edge as demonstrated in Fig. 5. The al-curve of pure PPP has a single peak near X N 340 nm. Previous studies [lo] assigned this peak to the PPP 7r--n*-band transition energy. In contrast, the n-PPP spectrum shows three peaks. The highest energy peak (X N 270nm) could be attributed to absorption by the polystyrene (PS) matrix due to its proximity to a prominent PS spectral feature and its disappearance in the spectrum of the fully converted PPP film. The peak near X N 320 nm could be identified as a blue-shifted PPP r-n* band transition energy. This assignment is based on earlier works [3, lo] which showed such blue-shift of the main optical absorption feature as the effective conjugation length of the polymer was reduced. It is the presence of the sub-gap feature around X N 420 nm
BLUE LIGHT-EMISSION
188
FROM A SEMICONDUCTOR
50
600
400
600
0 200
h(nm)
Fig. 5. Comparison of the photoluminescence and optical conductivity of nanostructured PPP (n-PPP) and pure PPP. that is unusual. The previous studies [3, lo] just mentioned did not reveal such sub-gap features. We suggest that the unique “quantum-dot” like structure of n-PPP give rise to this sub-gap absorption feature. In Fig. 5, we superimposed the PL spectra relative to the features in the absorption data of both the nanostructured and pure PPP samples. For pure PPP (dash-dot curves), the PL peak is Stokes-shifted from the main absorption peak. This behavior, typical of organic polymer semiconductors, has been attributed [I l] to the relaxation of the photo-excited electronhole quasiparticles to polaronic states within the (x-r*)-gap prior to radiative recombination. On the other hand, for n-PPP, the position of the PL peak coincides with the sub-(n-x*)-gap absorption feature. We infer from this result that direct radiative transition from the electronic sub-gap states induced by quantum confinement is responsible for the PL features of n-PPP. What we have just demonstrated is that n-PPP possesses optical characteristics which deviate from those of unstructured PPP. Below, we discuss two possible mechanisms which could account for these observed deviations. The first mechanism concerns states at the interface of n-PPP and polystyrene. Due to the significant increase of the surface-to-volume ratio in nanostructured systems, photon absorption due to optical transitions involving the interface states can be comparable to those of the bulk-states. Such optical transitions would account for the sub-gap spectral feature observed in n-PPP. Also, the localized nature of these interface states may explain the enhancement of the PL efficiency of the PPP quantum dots.
Vol. 95, No. 3
The second mechanism is related to the possibility of creating bound polaronic-exciton states in nanostructured organic semiconductors. A qualitative argument for this scenario is as follows. Recall that, from the neutron scattering data, the typical size of the n-PPP is between 4 and 10nm. Knowing that the monomer length in the phenyl group is about 0.4 nm, then a fully extended chain within the n-PPP will contain lo-25 monomer units. Since the delocalization length for a polaron [12] is typically a few monomer units, there is sufficient space for the formation of these quasiparticles within n-PPP. Moreover, since the delocalization length for a polaron is comparable to the size of n-PPP, it is plausible to expect an enhancement of the overlap between the quasiparticle wavefunctions. Thus, Coulomb interaction between the polaronic charge carriers is increased. This condition favors the formation of bound polaronic-exciton states. The sub-gap feature in the absorption spectra of n-PPP is attributed to transitions to these new electronic states induced by the spatial confinement. Also, the enhanced quasiparticle wavefunction overlap could explain the increase in the probability for radiative transitions which would account for the enhancement of the PL intensity in n-PPP. To conclude, the nanostructured organic semiconductor presented in this work revealed interesting new optical properties. It manifested an enhancement of the intensity and a blue-shift in the color of its emission characteristics. These improved emission properties could come from radiative transitions from unusual sub-gap electronic states induced by quantum confinement. By direct comparison, we have shown that these features depart from the luminescence and absorption characteristics of its unstructured counterpart. These modifications in their optical properties have useful implications towards the utilization of nanostructured organic semiconductors in light-emitting devices. - D.B.R. and L.Z. acknowledge the support for this work from the Optics Priority Program in Switzerland. G.W., M.R. and B.F. gratefully acknowledge the material help and advice from F. BovC, L. Noirez, L. Auvray, A. Brulet and J.P. Cotton. Acknowledgements
REFERENCES 1. 2. 3.
See the article of G.F. Neumark, R.M. Park & J.M. DePuydt, Physics Today 47,26 (1994) (and references cited). For a review, A.D. Yoffe, Adv. Phys. 42, 173 (1993). P.L. Burn et al., Nature 356,47 (1992).
Vol. 95, No. 3 4. 5. 6.
7. 8.
9.
BLUE LIGHT-EMISSION
FROM A SEMICONDUCTOR
G. Gustafsson et al., Nature 357, 477 (1992). G. Grem et al., Adv. Mater. 4, 36 (1992). B. Fraqois & X.F. Zhong, Synth. Met. 41-43, 955 (1991); X.F. Zhong & B. Francois, Makromol. Chem. 192, 2277 (1991). G.A. McConnell et al., Phys. Rev. Lett. 71,2102 (1993). L. Auvray & P. Auroy, Neutron, X-Ray and Light Scattering (Edited by P. Linderer & Th. Zemb). Elsevier (1991). D.B. Romero et al., Solid State Commun. 82, 183 (1992).
10.
11. 12.
189
G. Leising, K. Pichler & F. Stelzer, in Electronic Properties of Conjugated Polymers ZZZ(Edited by H. Kuzmany, M. Mehring & S. Roth) Vol. 91, p. 100. Springer-Verlag, Berlin (1989). R.H. Friend, D.D.C. Bradley & P.D. Townsend, J. Phys. D: Appl. Phys. 20, 1367 (1987). A.J. Heeger, S. Kivelson, J.R. Schrieffer & W.P. Su, Rev. Mod. Phys. 60,781 (1988).