Phonon Bottleneck Effect in Organic Molecules
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1172-T02-04
Phonon Bottleneck Effect in an Organic Molecule Supriyo Bandyopadhyay1 and Bhargava Kanchibotla Department of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, VA 23284, U.S.A. ABSTRACT We have measured the ensemble averaged transverse spin relaxation time T2* (associated with g = 4 resonance) in bulk powders of the organic molecule Alq3, and in samples containing 1-2 molecules confined in nanocavities of dimension ~ 2 nm. Both T2* times are strongly temperature dependent indicating that they are determined by phonon-mediated spin relaxation. Interestingly, the T2* time in nanocavities is ~2.5 times longer than in bulk powder over a wide temperature range. The longer T2* in the nanocavity is evidence of weakened electron-phonon interaction. We believe that electron-phonon interaction is suppressed because the cavity confines phonons and discretizes the phonon modes and phonon energies. As a result, the chances of a phonon induced (inelastic) spin relaxation event are reduced owing to the need to conserve energy in the relaxation process. This is a novel “phonon bottleneck effect” that to our knowledge has not been previously reported.
INTRODUCTION The concept of “phonon bottleneck effect” was first introduced in the context of electronic transitions between discrete energy levels in the conduction band of inorganic (semiconductor) quantum dots [1]. In a quantum dot, an electron’s energy in the conduction band is completely discretized so that the electron can occupy only certain allowed energy states known as subbands. When an electron decays from an excited state subband to the ground state subband, the excess energy is typically released in the form of a phonon and energy conservation will mandate that Eexcited − Eground = hω ph (1.1) where hω ph is the energy of the phonon emitted, Eexcited is the excited state energy and E ground is the ground state energy. This process is designated as phonon emission. Similarly, when an electron makes a transition from the ground state to an excited state, it must absorb a phonon to supply the missing energy. The processes of emission and absorption in a quantum dot are depicted in Fig. 1. In most semiconductor quantum dots, the only available phonon with sufficient energy to satisfy Equation (1.1) will be an optical phonon with fixed energy hωo . If the energy spacing between the discrete levels does not match this phonon energy, then Equation (1.1) cannot be satisfied and therefore the phonon emission and absorption processes are blocked or weakened by the failure to obey energy conservation. This weakening of the emission or absorption process is called the phonon bottleneck effect. Note that for this type of effect, electron confinement is required (to form discrete energy states or subbands), but phonon confinement is not required. 1
Corresponding author. E-mail: [email protected]
Eexcited phonon Eground
phonon
phonon Emission
Absorption
Figure 1: Phonon mediated emission and absorption processes within the conduction band
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