detecting single molecule absorption

Detecting singlemolecule absorption Martin Gruebele*, Joseph Lyding, Erin Carmichael, and Joshua Ballard University of Illinois, Urbana, IL 61801, USA *E-mail: gruebele@scs.uiuc.edu Laser excitation of surface-adsorbed molecules or nanostructures tunneling junction12, and STM detection affords about 20-fold better modulates the local electronic density of states, and can be spatial resolution than is possible with near-field optical techniques or detected by a scanning tunneling microscope with near-atomic related tip-enhanced techniques resolution Future applications range from single-molecule surface Optical excitation of surface adsorbates complements the several effects contribute to the observed signal: tip or substrate spatial resolution of scanning tunneling microscopy (STM) with chemical resolution Laser-induced electron tunneling from the carriers are photoexcited, laser heating disturbs the STM scan, and tip1, molecular electrons or vibrational modes are excited if the laser single-molecule fluorescence2, tip-enhanced Raman spectroscopy3, is on resonance, yielding the desired part of the signal Several ultrafast laser-induced dynamics4,5, photoexcited carrier dynamics6, techniques can be combined to enhance the absorption signal from and optically assisted patterning7 have been proposed or observed the molecular adsorbate or nanostructure over background noise9 by coupling STM and optical excitation8 However, absorption spectroscopy has proved elusive until now Single-molecule absorption spectroscopy detected by STM (SMA-STM), extends optical absorption techniques to imaging a single molecule on a surface9 The idea is very simple: on a semiconductor Excitation within the bandgap of the substrate, or a thin film on a transparent substrate, decreases substrate heating and thermal expansion Polarized excitation by total internal reflection reduces tip heating to a length scale below the optical wavelength, and reduces direct tip carrier excitation Laser amplitude and frequency modulation substrate, or other surface on which the electronic structure of a can be coupled with lock-in amplifier detection of the tunneling molecule or nanostructure possesses a relatively independent identity, current, so SMA images can be obtained in a single scan, alleviating tip optical pumping of the molecule to an excited state modifies the registration and thermal displacement problems We have shown that molecular wave function, and hence the local density of electronic narrowband frequency modulation can be used to eliminate heating states10 Molecular excitation thus changes the tunneling current and contributions because laser power remains constant, and can eliminate results in a different image when the laser wavelength is resonant with carrier effects because tip and surface absorption vary slowly with the molecular transition than when the laser is not resonant, or turned wavelength13 off entirely SMA-STM probes the molecule itself to detect absorption, Fig shows how the SMA-STM signal differentiates a carbon instead of probing a reduction in the transmitted number of photons, nanotube from a nearby surface defect Nanotubes were stamped in like conventional absorption measurements Laser excitation provides situ onto a hydrogen passivated Si(100) substrate with a fiberglass energetic or temporal selectivity, while the STM spatially resolves the applicator14 Laser excitation occurs within the substrate bandgap excitation at the submolecular level: the best of both worlds is available The 1251 nm, 1.2 mW laser is modulated at 0.84 kHz to obtain this to the experimenter SMA has certain advantages over other spatially signal, well within the >50 kHz response of the STM electronics9 The resolved spectroscopies Molecules absorb even in the presence of phase of the signal depends on the frequency modulation phase13, relaxation processes that quench fluorescence Relaxation of absorbates and is positive in Fig (bright signal compared with background) We can be slow enough11 such that optical absorption provides higher currently achieve dimer row resolution during laser illumination energy resolution than current-voltage (I-V) scans, as well as being less tip-dependent The tip further enhances the optical field at the 48 Implementation of SMA-STM requires careful instrumentation design9 When a laser illuminates a tip-adsorbate-surface junction, analysis to energy-transfer studies JUNE 2007 | VOLUME 10 | NUMBER We have demonstrated the surface analytic capabilities of SMASTM by using the sensitive diameter dependence of absorption ISSN:1369 7021 © Elsevier Ltd 2007 METHODS & MATERIALS (a) (b) (a) Fig A 7.4 nm-long carbon nanotube straddling a step edge on a Si(100) surface (a) SMA-STM image shows the signal from the nanotube, but only faint traces of signal from the substrate caused by residual thermal effects (b) Standard topographic STM image acquired simultaneously with the SMA scan Dimer rows, dangling bonds, and a surface defect (circled) are prominently visible7 (b) wavelength on tube diameter13 Differential absorption was detected via laser frequency modulation Thus, multiple nanotubes within the scan area can be differentiated spectroscopically even when their topographies are very similar We have developed quantitative models for the dependence of the absorption image on laser intensity and modulation13, as well as for the different systematic and random sources of noise in SMA-STM9 Noise sources include edge noise at topographic transitions dependent on tip velocity, thermal noise from rapid contraction and expansion of the laser-modulated junction, and mechanical noise caused by tipnanotube attractive interactions Fig Images showing the nonuniform SMA signal along a carbon nanotube (a) Topographic scan showing dangling bonds on the Si(100) surface and a carbon nanotube (b) Laser scan shows enhanced signal at the lower end of the tube, and complete interruption of the signal midway along the tube SMA-STM can also differentiate absorption sites within a single molecule Fig shows topographic and SMA-STM images of a Other applications for submolecular-resolution absorption carbon nanotube near a step edge surrounded by dangling bonds spectroscopy will be feasible in the near future: energy or electron The signal is particularly strong near the end of the tube, where the transfer between two nearby adsorbates could be detected by exciting I-V curve indicates metallic character (A catalyst particle from the a donor and monitoring an acceptor by STM Ultrafast two color manufacturing process could be present.) At a defect further up the pump-probe experiments could be used to study relaxation processes tube, visible as a slight kink in the topographic image, the absorption among adsorbates, or from adsorbates into the substrate or tip The signal disappears before picking up again I-V scans have recently use of nanometer metal films or other optically transparent conducting probed semiconductor-metal transitions in nanotubes15, as the I-V substrates will extend the technique from near-infrared to near- curve switches from a nonlinear to a linear relationship The signal ultraviolet excitation A bright future lies ahead for single-molecule shown in Fig 2b could be caused by such a metal-semiconductor optical absorption spectroscopy transition, or by interaction of the tube with a dangling bond The combination of I-V experiments and optical experiments will become a particularly powerful probe of optoelectronic molecular properties REFERENCES Acknowledgments Financial support from the National Science Foundation Chemistry Division, American Chemical Society, and Beckman Institute at the University of Illinois, Urbana-Champaign is gratefully acknowledged Grafström, S., J Appl Phys (2002) 91, 1717 Wu, S W., et al., Science (2006) 312, 1362 Ballard, J B., et al., Nano Lett (2006) 6, 45 Qiu, X H., et al., Science (2003) 299, 542 10 Tersoff, J., and Hamann, D R., Phys Rev B (1985) 31, 805 Anderson, N., et al., Nano Lett (2006) 6, 744 11 Guyot-Sionnest, P., et al., J Chem Phys (1995) 102, 4269 Bartels, L., et al., Science (2004) 305, 648 12 Bragas, A V., et al., Appl Phys Lett (1998) 72, 2075 Shigekawa, H., et al., Sci Technol Adv Mater (2005) 6, 582 13 Carmichael, E S., et al., J Phys Chem C (2007) 111, 3314 Maeda, Y., et al., Surf Sci (1997) 384, L896 14 Albrecht, P M., and Lyding, J W., Appl Phys Lett (2003) 83, 5029 Wong, V., and Gruebele, M., Chem Phys Lett (2002) 363, 182 15 Ruppalt, L B., and Lyding, J W., Small (2007) 3, 280 JUNE 2007 | VOLUME 10 | NUMBER 49 ... SMA-STM can also differentiate absorption sites within a single molecule Fig shows topographic and SMA-STM images of a Other applications for submolecular-resolution absorption carbon nanotube near... ultraviolet excitation A bright future lies ahead for single- molecule shown in Fig 2b could be caused by such a metal-semiconductor optical absorption spectroscopy transition, or by interaction... surface defect (circled) are prominently visible7 (b) wavelength on tube diameter13 Differential absorption was detected via laser frequency modulation Thus, multiple nanotubes within the scan