Molecular Tilting Alignment on Ag@C Nanocubes Monitored by Temperature-Dependent Surface Enhanced Raman Scattering

Abstract

Core@shell Ag@C nanocubes (NCs) with a cubic silver core (~60 nm of side length) and a coating of ultrathin amorphous carbon (~4 nm) have been synthesized on a large scale by a one-pot hydrothermal method. The carbon layer not only protects the Ag@C nanocubes from oxidation under hydrothermal condition, but also stabilizes the structure of Ag cores. Considering that optical properties of nanostructured metals strongly depend on the temperature for SERS measurement, in this work we systemically investigate the relationship between the orientation of molecules adsorbed on Ag@C NCs and temperature by SERS spectra. Results suggest that the adsorbed 4-MBA molecules prefer a flat orientation on the NC surface with temperature decreasing. In addition, Ag@C NCs after one-year storage in water still maintain high SERS-active capability. Our synthesized Ag@C NCs with excellent and stable optical properties can be potentially applied in the field of sensor and ultrasensitive spectral analysis.

Figure 1

(a) and (b) SEM images of Ag@C NCs dispersed onto Si substrate. (c) TEM image of a single Ag@C nanocube. The inset is the corresponding SAED. (d) HR-TEM image of Ag@C NC.

Figure 2

(a) STEM-EDS mapping of AgK in the Ag@C NC. (b) STEM-EDS of OK in the Ag@C NC. (c) STEM image of the Ag@C NC edge (d) the line STEM-EDS of AgK and OK in the Ag@C NC.

Figure 3

(a) Survey XPS spectrum of Ag NCs, (b) Ag3d, (c) O1s and (d) C1s fine XPS spectra of Ag NCs. The blue curves are obtained from the fresh samples and the red curves are from the aged one.

Figure 4

(a) and (c) SERS intensity maps at 1580 cm⁻¹ and 1080 cm⁻¹ of 4-MBA on the Ag@C nanocubes surface, respectively. (b) and (d) the intensity of the peaks in the above area spots.

Figure 5

(a) SERS spectra of Ag@C NCs at 10 K, 70 K, 130 K and 190 K, respectively. (b) The relationship between the SERS integrated peak area of band at 1580 cm⁻¹ depending on temperature which is represented by red line (left) and the relative intensity ratio of I_C-C/I_C-S at 1580 cm⁻¹ and 1080 cm⁻¹ as a function of temperature which is represented by blue line (right). The average relative integrated peak area was recorded from eight measurements on the substrate.

Figure 6

Schematic illustration for the electronic transition processing between the Ag@C NC and adsorbed tip molecule with decreasing temperature.

Figure 7

The effect of shelf life on the SERS enhancement: (a) 4-MBA(10⁻⁴ M)/Ag@C NCs; (b) 4-MBA(10⁻⁵ M)/Ag@C NCs; (c) 4-MBA(10⁻⁶ M)/Ag@C NCs; (d) 4-MBA(10⁻⁷ M)/Ag@C NCs.