Article pubs.acs.org/Langmuir
Morphological Control of PbS Grown on Functionalized SelfAssembled Monolayers by Chemical Bath Deposition Jing Yang and Amy V. Walker* Department of Materials Science and Engineering, University of Texas at Dallas, 800 W. Campbell Road, RL 10, Richardson, Texas 75080, United States S Supporting Information *
ABSTRACT: We have investigated the chemical bath deposition (CBD) of PbS on functionalized alkanethiolate self-assembled monolayers (SAMs) using time-of-flight secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. The deposition mechanism involves both cluster-by-cluster and ion-by-ion growth. The dominant reaction pathway and the chemical composition and morphology of the deposited layer are dependent on both the SAM terminal group and the experimental conditions. On −COOHterminated SAMs, three types of crystallites are observed: nanocrystals formed by heterogeneous ion-by-ion growth, larger needle-like particles, and ∼2 μm particles deposited by homogeneous cluster-by-cluster deposition. The nanocrystals nucleate at Pb2+-carboxylate surface complexes, and so strongly adhere to the substrate. On −OH- and −CH3-terminated SAMs, only the micrometer-sized particles are formed by a cluster-by-cluster deposition mechanism. These particles do not adhere strongly to the SAM surface and can be easily removed. SIMS and XPS analyses indicate that the larger needle-like crystals and micrometersized particles are composed of oxidized lead sulfide and lead oxides, while the nanocrystals are composed of ≥85% PbS. Using sonication-assisted CBD, we demonstrate that PbS is deposited by ion-by-ion growth alone on −COOH-terminated SAMs. The deposited film is more compact with a smaller grain size and is >90% PbS. complexes19 as well as the hydrophobic and hydrophilic properties of SAMs13 have been employed to control the morphology of ZnS19 and to perform selective growth of ZnS,13,19 CdSe,20 and PbS.13 The deposition efficiency of the CBD growth process can be modified by applying ultrasonication during the deposition.21−25 Ultrasonic vibrations cause the formation, growth, and implosive collapse of bubbles in liquids (acoustic cavitation). During cavitation, the bubble collapse produces hot spots, high pressures, and short lifetimes, which can drive chemical reactions.26 In the case of CBD, the application of ultrasound promotes ion-by-ion heterogeneous growth leading to changes in the deposit morphology.21,22 Oliva and co-workers23 also reported that sonication-assisted CBD changes the composition of CdS thin films. There have been few studies of sonication-assisted CBD. Little is known about the influence of ultrasound on the composition of the deposited film or the effect of the substrate on the deposition process. In this article, we examine the reaction pathways involved in the chemical bath deposition of PbS on functionalized alkanethiolate SAMs using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF SIMS), and electron microscopy. We demonstrate that the deposition mechanism involves both
1. INTRODUCTION Lead sulfide is a group IV−VI narrow bandgap semiconductor (Eg = 0.41 eV at room temperature)1 and is employed in the fabrication of mid-IR detectors.2 More recently, PbS has been used in solar cells,3 nonlinear optical devices,4 and thin film transistors.5 This is because PbS nanocrystals have a size tunable bandgap which can be adjusted to a few electronvolts.1 The significantly widening bandgap is associated with small effective masses of electrons and holes (me = mh = 0.09 mo) and a large exciton Bohr radius (18 nm).6 Thus, the optical and electrical properties of nanocrystalline PbS can be altered by adjusting the grain size.6,7 PbS nanostructures and thin films have been fabricated using a variety of wet and dry methods including molecular beam epitaxy,8 spray pyrolysis,9 electrodeposition,10 vacuum evaporation,2,11 atomic layer deposition,12 and chemical bath deposition (CBD).5−7,13−18 Chemical bath deposition is an attractive method for the deposition of chalcogenide thin films because it can be employed at low temperatures (≤50 °C) which are compatible with organic substrates, such as polymers and self-assembled monolayers (SAMs), and is inexpensive since the deposition is performed under ambient conditions. The morphology of films (including grain size and surface density) grown by CBD can be controlled using the experimental conditions, such as the reactant concentration, pH, and deposition temperature and time, as well as the nature of the substrate. For example, the formation of surface © 2014 American Chemical Society
Received: March 2, 2014 Revised: May 20, 2014 Published: May 22, 2014 6954
dx.doi.org/10.1021/la500784y | Langmuir 2014, 30, 6954−6962
Langmuir
Article
on top was then placed ∼50 mm from a 500 W Hg arc lamp equipped with a dichroic mirror and a narrow band-pass UV filter (280 to 400 nm) (Thermal Oriel, Spectra Physics Inc.). The sample was then exposed to UV light for 2 h to ensure that photooxidation was complete. The UV photopatterned −COOH-terminated SAM was then immersed in a freshly made 1 mM ethanolic solution of the second alkanethiol, HDT (−CH3-terminated SAM) for 24 h. In the areas exposed to UV light, the photooxidized −COOH-terminated SAM was displaced by HDT, and a −COOH/−CH3-patterned terminated SAM surface was then obtained. Finally, the patterned surfaces were rinsed copiously with degassed ethanol and dried with nitrogen gas. 2.4. Time-of-flight Secondary Ion Mass Spectrometry. Timeof-flight secondary ion mass spectrometry data were acquired on an ION TOF IV spectrometer (ION TOF Inc., Chestnut Hill, NY) equipped with a Bi liquid metal ion gun. Briefly, the instrument consists of a load lock for sample introduction and preparation and analysis chambers each separated by a gate valve. The pressure of the preparation chamber was maintained at ≤7 × 10−9 mbar, while the analysis chamber was maintained at ≤5 × 10−9 mbar. The primary Bi+ ions had a kinetic energy of 25 keV and were contained in an ∼100 nm diameter probe beam. The beam was rastered over a (100 × 100 μm2) area during spectra acquisition and a (500 × 500 μm2) area during image acquisition. All spectra were acquired within the static regime30 using a total ion dose less than 1010 ions cm−2. The secondary ions were extracted into a time-of-flight mass spectrometer using a potential of 2000 V and were reaccelerated to 10 keV before reaching the detector. For each experimental condition, at least three samples were prepared, and three areas on each sample were examined. The peak intensities were reproducible to within ±15% from scan-to-scan and from sample-to-sample. Optical images of the samples were obtained using a video camera (ExwaveHD, Sony) mounted in the analysis chamber. 2.5. Scanning Electron Microscopy (SEM). Scanning electron microscopy measurements were performed on a dual-beam FIB instrument (Nova 200 Nanolab, FEI Company) and a Zeiss LEO Supra 40 FESEM. The Nova 200 Nanolab has an electron beam energy up to 30 keV, and the Zeiss LEO Supra 40 has an operating voltage of 5 kV. 2.6. X-ray Photoelectron Spectroscopy (XPS). Photoelectron spectra were measured with a PerkinElmer 5600 ESCA system and a PHI VersaProbe II (Physical Electronics Inc.) each equipped with a monochromatic Al Kα X-ray source (Ep = 1486.7 eV). Typically, the pressure of the chambers was
