We present a high-resolution distance dependence study of surface-enhanced Raman scattering (SERS) enabled by atomic layer deposition (ALD) at 55 and 100 °C. ALD is used to deposit monolayers of Al2O3 on bare silver film over nanospheres (AgFONs) and AgFONs functionalized with self-assembled monolayers. Operando SERS is used to measure the intensities of the Al-CH3 and C-H stretches from trimethylaluminum (TMA) as a function of distance from the AgFON surface. This study clearly demonstrates that SERS on AgFON substrates displays both a short- and long-range nanometer scale distance dependence. Excellent agreement is obtained between these experiments and theory that incorporates both short-range and long-range terms. This is a high-resolution operando SERS distance dependence study performed in one integrated experiment using ALD Al2O3 as the spacer layer and Raman label simultaneously. The long-range SERS distance dependence should make it possible to detect chemisorbed surface species located as far as ∼3 nm from the AgFON substrate and will provide new insight into the surface chemistry of ALD and catalytic reactions.

This paper describes how the ability to tune each nanoparticle in a plasmonic hetero-oligomer can optimize architectures for plasmon-enhanced applications. We demonstrate how a large-area nanofabrication approach, reconstructable mask lithography (RML), can achieve independent control over the size, position, and material of up to four nanoparticles within a subwavelength unit. We show how arrays of plasmonic hetero-oligomers consisting of strong plasmonic materials (Au) and reactant-specific elements (Pd) provide a unique platform for enhanced hydrogen gas sensing. Using finite-difference time-domain simulations, we modeled different configurations of Au–Pd hetero-oligomers and compared their hydrogen gas sensing capabilities. In agreement with calculations, we found that Au–Pd nanoparticle dimers showed a red-shift and that Au–Pd trimers with touching Au and Pd nanoparticles showed a blue-shift upon exposure to both high and low concentrations of hydrogen gas. Both Au–Pd hetero-oligomer sensors displayed high sensitivity, fast response times, and excellent recovery.

In this work, we report synthesis strategies to produce Ag nanoparticles by AB-type and ABC-type atomic layer deposition (ALD) using trimethylphosphine-(hexafluoroacetylacetonato) silver(I) ((hfac)Ag(PMe 3 )) and formalin (AB-type) and (hfac)Ag(PMe 3 ), trimethylaluminum, and H 2 O (ABC-type). In situ quartz crystal microbalance measurements reveal a Ag growth rate of 1−2 ng/cm 2 /cycle by ABC-type ALD at 110°C and 2−10 ng/cm 2 /cycle for AB-type ALD at 170−200°C. AB-type Ag ALD has a nucleation period before continuous linear growth that is shorter at 200°C. Transmission electron microscopy reveals that AB-type Ag ALD particles have an average size of ∼1.8 nm after 10 cycles. ABC-type Ag ALD particles have an average size of ∼2.2 nm after 20 cycles. With increasing ALD cycles, ABC-type Ag ALD increases the metal loading while maintaining the particle size but AB-type Ag ALD results in the formation of bigger particles in addition to small particles. The ability to synthesize supported metal nanoparticles with well-defined particle sizes and narrow size distributions makes ALD an attractive synthesis method compared to conventional wet chemistry techniques. ■ INTRODUCTIONThe ability to control the size, structure, and chemical composition of noble metal nanoparticles (NPs) is an ongoing endeavor in catalysis research. Noble metal NPs are of interest because of their unique chemical and optical properties. It is well-known that NPs can differ chemically from bulk materials owing to the reduced coordination numbers and high percentage of surface atoms 1 and should be tunable through better control of their size, shape, and structure. 2 Supported silver (Ag) NPs are an example of noble metal NPs that have been studied extensively. As a material, Ag is of interest due to its high electrical and thermal conductivity and its high optical reflectivity. 3 Ag is also used in catalytic and plasmonics applications. In plasmonics, the localized surface plasmon resonance (LSPR) on Ag NPs leads to strongly enhanced local electric fields, which can be used to enhance Raman scattering in surface-enhanced Raman spectroscopy (SERS). 4−7 The LSPR of Ag NPs can also be used to enhance fluorescence, 8 luminescence, 9 and photoabsorption. 10 In catalysis, Ag NPs are used mainly as catalysts in the epoxidation of ethylene to form ethylene oxide (EO). 11 EO is used to make engine antifreeze, ethoxylates, plastics, and higher glycols. 12−15 Recently, small alumina-supported Ag clusters (∼3.5 nm) have been shown to be very active for direct propylene epoxidation to propylene oxide at low temperatures, with negligible amounts of CO 2 formation. 16,17 Conventional methods of preparing Ag catalysts include wet impregnation, coprecipitation, colloidal synthesis, ion-exchange, and chemical vapor deposition (CVD). 2,18 These methods are reliable at producing catalytic materials but oftentimes NPs with a broad size distribution are formed. It remains a challenge to synthesize small Ag NPs with narrow size distributions on oxide supports. 18 At...

This work demonstrates for the first time the capability of measuring surface vibrational spectra for adsorbates during atomic layer deposition (ALD) reactions using operando surface-enhanced Raman spectroscopy (SERS). We use SERS to study alumina ALD growth at 55 °C on bare silver film-over nanosphere (AgFON) substrates as well as AgFONs functionalized with thiol self-assembled monolayers (SAMs). On bare AgFONs, we observe the growth of Al−C stretches, symmetric C−H and asymmetric C−H stretches during the trimethylaluminum (TMA) dose half-cycle, and their subsequent decay after dosing with H 2 O. Al−C and C−H vibrational modes decay in intensity with time even without H 2 O exposure providing evidence that residual H 2 O in the ALD chamber reacts with −CH 3 groups on AgFONs. The observed Al−C stretches are attributed to TMA dimeric species on the AgFON surface in agreement with density functional theory (DFT) studies. We observe Al−C stretches and no thiol vibrational frequency shifts after dosing TMA on AgFONs functionalized with toluenethiol and benzenethiol SAMs. Conversely, we observe thiol vibrational frequency shifts and no Al−C stretches for AgFONs functionalized with 4-mercaptobenzoic acid and 4-mercaptophenol SAMs. Lack of observed Al−C stretches for COOH-and OH-terminated SAMs is explained by the spacing of Al−(CH 3 ) x groups from the SERS substrate. TMA penetrates through SAMs and reacts directly with Ag for benzenethiol and toluenethiol SAMs and selectively reacts with the −COOH and −OH groups for 4-mercaptobenzoic acid and 4-mercaptophenol SAMs, respectively. The high sensitivity and chemical specificity of SERS provides valuable information about the location of ALD deposits with respect to the enhancing substrate. This information can be used to evaluate the efficacy of SAMs in blocking or allowing ALD deposition on metal surfaces. The ability to probe ALD reactions using SERS under realistic reaction conditions will lead to a better understanding of the mechanisms of ALD reactions.