Patent Application
20140036262
Raman spectroscopy is used to study the transitions between molecular energy states when incident photons scatter as a result of their interaction with an analyte (i.e., a species, molecule or, in general, matter being analyzed). The scattered photons have an energy that is shifted in frequency due to two processes: the incident photons excite the analyte to cause the analyte to transition from a certain initial energy state to another (either virtual or real) energy state; and the excited analyte radiates as a dipole source to produce a scattered signal. The analyte radiates under the influence of its environment at a frequency that may be relatively low (called Stokes scattering), or relatively high (called anti-Stokes scattering), as compared to the frequency of the excitation photons.
The Raman spectra of a given analyte have characteristic peaks corresponding to the Raman-active vibrational modes (including bending, stretching, twisting modes), which may be used to identify the analyte. As such, Raman spectroscopy is a useful technique for a variety of chemical or biological sensing applications. However, the intrinsic Raman scattering process is often relatively inefficient. For purposes of improving the efficiency of the above-described excitation and radiation processes, enhancements may be made using surface enhanced Raman spectroscopy (SERS). These enhancements typically include rough metal surfaces, metal nanoparticles various types of nano-antennas, nanostructures such as nanowires coated with metal, black silicon coated with metal, as well as waveguiding structures.
Techniques and systems are disclosed herein for purposes of both electronically and plasmonically enhancing a Raman signal that is produced by the Raman scattering of incident photon energy (herein called a “pump signal”) by a target sample (i.e., a species, molecule(s) or, in general, matter being analyzed and herein called an “analyte”). More specifically, in accordance with example implementations that are disclosed herein, a surface enhanced Raman spectroscopy (SERS) sensor has an integrated structure that contains a plasmonically enhancing material and an electronically enhancing material.
The electronically enhancing material may be a semiconductor or any other material that may be optically excited (such as an organic dye, rhodamine 6G, in a polymer host material, such as polyimide, for example) such that the material may be optically pumped to an upper excited state by radiation, and when the excited state relaxes to a lower state, such as the ground state, energy is transferred from the material to the analyte. The electronically enhancing material may be disposed below or above the plasmonically enhancing material of the integrated structure, depending on the particular implementation.
The plasmonically enhancing material may be any material that gives rise to surface plasmons that enhance the electric field surrounding the material when the analyte is placed in proximity (within 10 nanometers (nm), for example) of the material. For specific examples that are disclosed herein, the plasmonically enhancing material may be a metal, such as palladium, platinum, aluminum, copper, gold, silver or nickel, or a combination of two or more of these metals. Other plasmonically enhancing materials (other metals and dielectric materials, for example) may be used, in further implementations.
For the specific examples that are disclosed herein, the plasmonically enhancing material partially or completely overlays (or underlays) the electronically enhancing material, such as example implementations disclosed herein in which a partial coverage and/or semitransparent plasmonically enhancing metal is disposed on an electronically enhancing material. However, in further implementations, the electronically enhancing material may be disposed on the plasmonically enhancing material. In an example implementation, the plasmonically enhancing material may be a plasmonic metal, such as gold; and the electronically enhancing material may be an organic dye, such as rhodamine 6G, which is disposed on top of the gold in a polymer host, such as polymide.
In this context, “on a structure” or “on a material” means at least partially supported by the structure/material, which may or may not involve contact with the structure/material. For example, a plasmonically enhancing material that is disposed on an electronically enhancing material may or may not contact the electronically enhancing material (i.e., no, one or multiple intervening layers may be disposed between the materials, for example), depending on the particular implementation.
For example implementations described herein, the electronically enhancing material is formed from an underlying structure, and the plasmonically enhancing material is semi-transparent and/or partially covers the electronically enhancing structure. In this regard, the plasmonically enhancing material is sufficiently thin or patterned to allow communication through the plasmonically enhancing material in the frequencies of interest, such as the frequencies of the spectra associated with the incident pump signal (635, 785, 850, 980, 1300 or 1550 nanometers (nm), to name a few possible pump wavelengths) and the Raman signal (typically with Raman shifts of 100 to 3000 centimeters (cm)−1). As a more specific example, in accordance with some implementations, the plasmonically enhancing material may have a thickness of less than or equal to 100 nm, although the plasmonically enhancing material may have a thickness greater than 100 nm. For example, in some implementations, a relatively thicker plasmonically enhancing material may be employed, which has openings (a “mesh” or random islands, for example), such that the partial coverage of the plasmonically enhancing material allows communication of the frequencies of interest through the openings. The plasmonically enhancing material may be formed by a layer fabrication process suitable for forming a relative thin layer, such as a process that involves atomic layer deposition (ALD), sputtering, angle evaporation, for example.
In accordance with example implementations, the electronically enhancing material is part of a nanostructure. In general, “nanostructure” refers to a structure that has at least one dimension that is on the nano-scale (from 1 nm to 1000 nm, for example). The nanostructure may, in general, be a semiconductor, such as a Group II-VI semiconductor (i.e., a semiconductor formed from an element selected from Group II of the periodic table and an element selected from Group VI of the periodic table) or a Group III-V semiconductor (i.e., a semiconductor formed from an element selected from Group III of the periodic table and an element selected from Group V of the periodic table). The nanostructure may be formed from other materials, in accordance with other implementations.
For the example that is depicted in
In accordance with example implementations, a spatially repeated or randomly distributed structure is integrated with the base substrate 20 and includes an underlying electronically enhancing nanostructure and a plasmonically enhancing metal is disposed on the nanostructure. The nanostructure may, in general, may be a semiconductor material, such as a material selected from the Group II-V family of elements or the Group III-V family of elements in the form of quantum dots or nanowires, in accordance with example implementations.
In accordance with an example implementation, the nanostructure is a quantum dot structure 30. As depicted in
Referring also to
To form the quantum dots 50, a Group III-V or Group II-VI semiconductor (as examples) may be grown epitaxially, or synthesized separately and spun onto the base substrate 20 in a resist-type material (non-limiting examples of which include polyamide, a spin-on glass, photoresists, or the like). As a more specific example, the quantum dot 50 may be formed from a semiconductor such as GaN, InGaN, AlGaN, GaAs, AlGaAs, InP, InGaAs, InAlAs, InGaAsP, in which interband transition occurs. In other implementations, the quantum dot 50 may be formed from a semiconductor structure, such as an InGaAs/InAlAs semiconductor structure, in which quantum cascade intraband transition occurs. Thus, many variations are contemplated, which are within the scope of the appended claims.
The metal layer 40 is a semi-transparent layer, in accordance with example implementations, which means that the metal layer 40 has a thickness T (a thickness T less than 100 nanometers, for example) that is thin enough to allow the spectra of the Raman and pump signals to pass through the layer 40 or greater than 100 nm in case of partial metal coverage.
In accordance with some implementations, the metal layer 40 may be deposited using atomic layer deposition (ALD), (sputtering, angle evaporation), and the ALD may be used to deposit the metal layer as a single metal layer across all of the quantum dots 50 of the sensor 10, as depicted in
SERS sensors in accordance with further implementations may include nanostructures other than quantum dots for purposes of electronically enhancing the Raman signal. For example, referring to
Thus, referring to
Other variations are contemplated, which are within the scope of the appended claims. For example, in accordance with further implementations, the quantum dot structure 30 or nanowire structure 60 may include an integrated resonator to increase the optical gain in the quantum dot or semiconducting nanowires to allow energy transfer from the semiconductor to the analyte, energy transfer from the semiconductor to the plasmon and/or enhancement of the Raman emission process. In general, the resonator improves the Q, or the optical intensity, in the quantum dots or nanowires, which increases the optical gain of the material. For example, as depicted in
The Bragg mirror includes overlapping layers, such as overlapping layers 212, 214 and 216 (depicted as examples in
Depending on the particular implementation, the layers 212, 214 and 216 may be, as examples, dielectric layers, silicon nitride layers and/or silver layers. The layers 212, 214 and 216 may be epitaxially deposited (by atomic layer deposition (ALD), for example) for purposes of conforming to the underlying nanostructure, such as the nanowire 204. As depicted in
Bragg mirrors may be formed on nanostructures other than quantum dots, in accordance with further implementations. For example,
A resonator other than a Bragg mirror-based resonator may be used in a SERS sensor, in accordance with further implementations. For example, referring back to
The sensor 10 may have features other than those described above to further enhance a spectral bandwidth of the Raman signal by varying the sizes and/or compositions of the nanostructures. In this manner, the electronic enhancement is, in general, a function of, or is dependent upon, the bandgap of the semiconductor and geometry of the electronically enhancing structure. Therefore, by incorporating a range of differently-sized nanostructures and/or incorporating a range of nanostructure having different compositions the spectral bandwidth of the semiconductor nanostructures can match approximately the spectral bandwidth of the Raman signal of the analyte.
For example, as depicted in
In further implementations, the compositions of the nanostructures may be varied for purposes of expanding the enhanced Raman bandwidth. For example, quantum dots have varying semiconductors and/or semiconductor structures may be spatially distributed across the surface of the substrate 20. As a more specific example, some of the quantum dots may be formed from GaAs that has an electronic resonance near an 800 nm wavelength, as blue shifted by a few or few hundred nanometers, depending on the size of the nanostructure; and other quantum dots may be formed form InP that has an electronic resonance near a 900 nm wavelength, as blue shifted by a few or few hundred nanometers, depending on the size of the nanostructure. Collectively, quantum dots having such varying compositions present an effective electronic resonance bandwidth that expands the portion of the Raman bandwidth that is electronically enhanced. In further implementations, the compositions of nanostructures other than quantum dots may be varied for purposes of expanding the enhanced Raman bandwidth.
Thus, referring to
While a limited number of examples have been disclosed herein, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations.
