Article pubs.acs.org/JPCC
Transparent Metal Films for Detection of Single-Molecule Optical Absorption by Scanning Tunneling Microscopy Lea Nienhaus,†,‡ Gregory E. Scott,†,‡ Richard T. Haasch,§ Sarah Wieghold,∥ Joseph W. Lyding,‡,⊥ and Martin Gruebele*,†,‡,# †
Department of Chemistry, ‡Beckman Institute, §Frederick Seitz Materials Research Laboratory, ⊥Department of Electrical and Computer Engineering, and #Department of Physics, University of Illinois, Urbana, Illinois 61801, United States ∥ Department of Chemistry, Technische Universität München, Lichtenbergstr. 4, 85748 Garching, Germany S Supporting Information *
ABSTRACT: Atomically flat, conductive, and transparent noble metal films are produced to extend the wavelength range of room-temperature singlemolecule optical absorption detected by scanning tunneling microscopy (SMASTM). Gold films grown on a platinum underlayer to 15 nm total thickness, deposited by electron beam evaporation onto c-plane sapphire substrates, show sufficient light transmission for backside illumination for laser-assisted STM experiments. Low resistance, transparency, and the atomically flat island surfaces make these good substrates for SMA-STM studies. Monte Carlo lattice kinetics were simulated to allow for a better understanding of the growth modes of the Pt−Au films and of the achieved morphologies. SMA-STM is detected for a quantum dot deposited by aerosol spraying onto Pt−Au films, demonstrating the suitability of such films for single-molecule absorption spectroscopy studies.
■
INTRODUCTION Optical absorption is a “universal” detection technique: it works even in highly dissipative environments where emission is quenched. Single-molecule optical absorption was first observed at liquid helium temperature in the wings of spectral lines.1 Recently, a number of techniques have been developed to detect optical absorption of single chromophores at room temperature,2−5 including single-molecule absorption detected by scanning tunneling microscopy (SMA-STM), which can localize the absorption signal with subnanometer resolution.6,7 Two-photon pump−probe techniques have also been used to infer absorption transitions by ground-state depletion,8,9 although these multilaser experiments are not linear optical probes of absorption. SMA-STM requires a transparent, conductive, and atomically smooth substrate for the chromophore. Laser excitation of the chromophore changes the local electronic density of states, either by populating the chromophore’s excited state or through subsequent relaxation and local heating.10 The resulting change in tunneling current is then detected.6 To increase the signal-to-noise ratio, laser modulation and lock-in detection of the current are used; rear illumination and total internal reflection in the transparent substrate minimize tip heating; the sharp tip increases the intensity of the evanescent wave absorbed by the chromophore; and frequency modulation can be used to keep laser power on the substrate constant to minimize thermal distortion.10 Previous studies of SMA-STM were performed by backside illumination of hydrogen-passivated silicon surfaces.7 The © 2014 American Chemical Society
backside illumination technique and the Si optical bandgap of 1.1 eV limited the possible molecules to having an absorption maximum in the near-infrared. Thin metal films could extend such studies into the visible and near-UV region. Gold films on sapphire previously developed by Carmichael et al.11 were transparent and conductive but still had a high roughness at the atomic level (0.4 nm rms roughness). While large nanostructures such as carbon nanotubes could be imaged on such a surface, smaller chromophores, such as quantum dots and organic molecules, could not be identified in previous work. To extend single-molecule optical absorption to the visible spectrum, here we develop hybrid films with gold on a platinum underlayer. The growth mode of gold on platinum instead of on sapphire produces films that are very flat, allowing detection of small chromophores such as quantum dots. The films are sufficiently transparent in the visible for rear illumination and also meet the requirement for STM of being conductive. While the formation of self-assembled monolayers (SAMs) has been reported for platinum surfaces,12 we were interested in developing a gold-terminated film to facilitate future observation of thiol-derivatized molecules or biomolecules on the many types of SAMs developed for gold surfaces. We describe in detail the growth conditions for platinum and platinum−gold hybrid films. We present lattice simulations13 to model the surface growth patterns. The simulations agree with the Received: February 20, 2014 Revised: May 21, 2014 Published: May 22, 2014 13196
dx.doi.org/10.1021/jp501811z | J. Phys. Chem. C 2014, 118, 13196−13202
The Journal of Physical Chemistry C
Article
Figure 1. (2 μm)2 AFM images of 15 nm deposition of Pt on c-plane sapphire at ca. 370 °C, 761−799 °C, and 899 °C (left to right), showing the trend in the growth with increasing temperature. 500 nm scale bars.
peak, 1315 nm) are applied ex situ by aerosol deposition using an airbrush (Iwata SM-SB) connected to a clean nitrogen gas source at 35 psi. PbS quantum dot stock solution (5 μL; 10 mg/mL in toluene) is diluted in 1 mL of toluene. The solution (200 μL) is given into the airbrush and pulsed onto the sample at a distance of 10 cm; the sample is allowed to fully dry between pulses. AFM and STM Setup. Atomic force microscopy (AFM) measurements are performed in air on a Bruker AFM, using 300 kHz silicon cantilevers in tapping mode. STM measurements are performed on a home-built ultrahigh-vacuum STM similar to ones previously reported,14 using electrochemically etched tungsten tips for preliminary imaging and mechanically cut platinum−iridium tips (80:20) for SMA-STM imaging of quantum dots on our surfaces. Images are collected at tunneling currents of 5−100 pA and a typical sample bias of 2 V. For SMA-STM experiments, the previously reported rear illumination technique is applied.7 A small glass prism couples the laser beam into the rear face of the sapphire substrate. The beam then undergoes total internal reflection at the front face, where the thin film and quantum dot sample are scanned by the STM tip. A 532 nm diode pumped solid state (DPSS) laser (Thorlabs) is amplitude-modulated at 2.2 kHz by a mechanical chopper wheel (Thorlabs) whose reference output is fed into a lock-in amplifier. The average tunneling current and its component at the laser modulation frequency are measured simultaneously to yield a current topographic image as well as the X and Y phase images of the modulated current detected by the lock-in amplifier. XPS Setup. XPS measurements are carried out using a Kratos Axis ULTRA XPS system. Angle-resolved XPS at takeoff angles of 15°, 30°, 45°, and 90° is performed using X-rays from a monochromatic X-ray source (Al Kα) and a pass energy of 160 eV. Modeling. Kinetic simulations of the deposition process are carried out with a simple square lattice model on an s2 = 100 × 100 grid, as described by Xiao and Ming13 and in our previous studies (see Supporting Information for code and Figures S2 and S3 for additional simulations).11 All energies and temperatures are in units of kT0 (room temperature). The model randomly deposits metal particles on the grid and allows them to diffuse and interact. The rate of depositions per time step is D. The number of particles diffusing per time step Δt is s2 exp(−Ea,diff(T)/T). Ea,diff(T) is the effective activation energy for diffusion to an adjacent site. Only single steps forward, backward, left, or right are allowed, with the particle having to occupy the lowest unoccupied position at the adjacent site. The energy of a particle is E(t)=nEpp+mEps, where n ranges from 1 to 6 neighboring particles and m from 0 to 1 depending on
experiments in that rough and nonconductive Pt underlayers form smooth conductive terraces when coated with a 10 nm layer of gold. Finally, we demonstrate that on these surfaces an individual quantum dot can be successfully imaged by STM and that its optical absorption can be detected by SMA-STM. Absorption of visible light is resolved to the quantum dot size of 4 nm, making this the smallest structure resolved by optical absorption.
■
METHODS Thin Film Deposition. We use double-side polished cplane sapphire wafers of 0.43 mm thickness as the film substrate (University Wafer). Prior to deposition these are degreased by sonication for 30 min each in acetone, isopropanol (IPA), and methanol. The substrates are then annealed at 1000 °C in air for 12 h. The metal films are deposited by electron beam (ebeam) evaporation from >99.99% purity noble metal sources using a CHA SEC-600 E-Beam/Thermal Evaporator at base pressures of 10−6 Torr in a class 100 cleanroom environment. Film thickness is determined using a 6 MHz gold-plated quartz crystal monitor (INFICON), which determines only the average thickness of the deposited metal. As a result, the actual thickness of our porous films is slightly higher than the reported value. Temperature control is achieved by mounting the sample on the face of a button heater (Heatwave Laboratories, see Figure S1 of Supporting Information for geometry), utilizing existing power and thermocouple feedthroughs into the vacuum chamber. The temperature is monitored by a home-built thermocouple assembly, which is mounted on the backside of the button heater. Because the thermocouple is located on the backside of the button heater, there will be a discrepancy between the actual substrate temperature and the measured temperature of the button heater. We verified this by comparing thermocouple values with direct readings of the sample surface using a pyrometer, which read 20−30 °C lower at ≈800 °C. We report thermocouple values here. Sample Preparation. The films are used as fabricated, with no further cleaning or annealing steps. To ensure better contact between the sample holder and the thin films, thick silver contacts are deposited at the sample edge using colloidal silver paint (Ted Pella Inc.). To allow transmission of the laser beam through the back face of the sapphire wafer for total internal reflection at the front face, a 3 mm fused silica right angle prism is glued to the back side using a transparent ultrahigh-vacuum (UHV) compatible epoxy (Epotek). Before STM imaging, all samples undergo a 12 h, 120 °C degas in UHV. Lead sulfide quantum dots (Evident Technologies; diameter, 4.2 nm excluding shell; absorption peak, 1228 nm; emission 13197
dx.doi.org/10.1021/jp501811z | J. Phys. Chem. C 2014, 118, 13196−13202
The Journal of Physical Chemistry C
Article
Figure 2. Simulated growth of a platinum film for varying temperatures and deposition rates in a 100 × 100 lattice model. Each coarse-grained site corresponds to ≈6 nm in our experiments. The deposition rate D is increased from top to bottom, and the temperature T from left to right. D is in units of (Δt)−1, where Δt is the time required on average for a particle to diffuse one lattice point. E and T are in units of kT0, where T0 is room temperature. In these simulations, the particle−particle energy changes with temperature (Epp(x/kT0) = (4x − 6) kT0) and the particle−surface energy is constant at Eps = +1. The diffusion barrier Dbar is also changed with increasing temperature: Dbar(x/kT0) = −8x/3 + 12kT0. The empirical parameter values and the code are given in Supporting Information.
deposition. This became the new fixed surface for further deposition of Au (with its own values of Ea,diff, Epp, and Eps). This very simple model was able to account for all observed surface morphologies.
whether the particle is sitting on the underlying surface or on another particle. Epp is the effective particle−particle adhesion energy, and Eps is the effective particle−surface adhesion energy. The Metropolis algorithm is used to decide whether a particle diffuses from one time step to the next: if E(t + Δt) < E(t), it is moved; otherwise, comparison of the Boltzmann factor relative to a random number between 0 and 1 is used to decide whether to move the particle or not.15 In our previous work, the particles corresponded to single Au atoms.11 Here, to keep the simulation size small, we adjusted the effective diffusion coefficient of particles downward by increasing Ea,diff and the effective interaction energy Epp between particles upward to correspond to ca. 6 ± 2 nm per lattice point, or particles ca. 20 atoms on a side. To simulate deposition of Au on Pt, the Pt model was annealed by allowing diffusion without
■
RESULTS Pt Sublayer Film. The first objective was to reproduce and fine-tune results by Braunschweig et al.16 to produce locally flat conductive platinum films. Figure 1 shows (2 μm)2 AFM images of the temperature dependence of the growth of 15 nm platinum films on c-plane sapphire. Deposition at too low temperatures (about 370 °C) results in rough continuous films, which do not allow for unambiguous detection of small molecules on the surface. An increase of the temperature up to a large range between ca. 750−830 °C allows for higher 13198
dx.doi.org/10.1021/jp501811z | J. Phys. Chem. C 2014, 118, 13196−13202
The Journal of Physical Chemistry C
Article
Figure 3. Top: comparison of AFM images of a 15 nm Pt film (left panel), a 5 nm Pt sublayer (center panel), and a 5 nm Pt + 10 nm Au film (right panel). Bottom: the corresponding films simulated on the 100 × 100 lattice model. T = 3.75 and D = 2 for the Pt surfaces, with interaction energies as in Figure 2. The Pt was annealed at T = 3.5 for 10 000 time steps Δt. T = 2.5 and D = 8 for the Au overlayer, with interaction energies EAu−Au= −8 and EAu−Pt = +1, yielding a relative energy difference per particle face of +9kT0 for Au binding Pt versus Au binding Au. The size of one particle in the coarse-grained lattice model is ca. 6 nm, and the bottom row of images has been scaled accordingly.
merge into larger islands and finally into a continuous film with increasing average thickness before growing in apparent height. Pt−Au Hybrid Film. To facilitate thiol bonding for compatibility with SAMs or thiol-derivatized small molecules, we developed a film that was gold-terminated. Attempts of growing pure gold films on sapphire resulted in large pillar growth and therefore nonconductive films at elevated temperature, or rough films (>0.5 nm rms roughness) at room temperature. Unlike for Pt films, a compromise temperature and deposition rate could not be found for pure gold films. Inspired by previous work on niobium seed underlayers,18 we developed a hybrid film made of a Pt underlayer as described above, covered by a Au layer. A 5 nm Pt seed layer followed by 10 nm gold deposition helped us overcome the conductivity− roughness trade-off and resulted in very flat and conductive films with low roughness (
