Terahertz-based nanometrology: multispectral imaging of nanoparticles and nanoclusters in suspensions
Silica nanoparticles suspended in an organic solvent (nanosuspension) have been imaged and characterized via terahertz nanoscanning reconstructive three-dimension (3D) imaging technique. The size of individual silica nanoparticles in the suspension was quantified. In addition, the presence of nanoclusters along with their distribution in the suspension was visualized in 3D. It has also been qualitatively demonstrated that the volume fraction of solvent is significantly higher than that of the silica nanoparticles; an observation consistent with the composition of the nanosuspension in the present investigation. The measured size range of individual nanoparticles was found to be 10–12 nm, while the manufacturer’s specification indicates a nanoparticle size distribution in the range of 10 to 15 nm. However, a typical nanocluster size was determined to be 17.5 nm, thus indicating the presence of nanoparticles less than 10 nm. The nanometrology instrument used in this investigation was based on a dendrimer dipole excitation-based continuous wave terahertz source generating > 200 mW stable terahertz power.
KeywordsTerahertz multispectral reconstructive imaging Nanoslurry Silica nanoparticles Nanoclusters Nanosuspension Nanoscale instrumentation
Characterization of nanoparticles in suspension, or as “nanosuspensions” is important in a wide range of areas such as; drug delivery, lubricants, as well as abrasion and polishing protocols. More specifically, nanosuspensions are used for fine scale polishing in the semiconductor industry. Such suspensions are commonly referred to as “slurries,” wherein, silica nanoparticles are suspended in a suitable solvent for the polishing process known as the chemical and mechanical polishing (CMP) protocol. Many different combinations of nanoparticles and suspending media are used depending on the formulation and polishing specifications. However, a common problem in all cases, is to measure the size of the nanoparticles while they are still in the suspension. Many existing methods require extensive optimization, especially for new types of nanoparticles and solvent combinations. Some insights may be gained from the fields of environmental chemistry of natural nanomaterials and from fundamental colloid chemistry (Hassellöv et al. 2008; Wang et al. 2009). A critical requirement for these polishing slurries is that the nanoparticles must remain in suspension without agglomeration since particle aggregation will change the polishing characteristics.
Recently, we reported a method for measuring the size of quantum dots (Rahman et al. 2016a) and carbon nanotubes (CNTs) (Rahman 2016). These nanoparticles were also in solution, but they were first spun onto a silicon wafer. Using this spinning protocol, the nanoparticles formed layers of specific thickness that are determined mainly by the solution concentration and casting speed. Following the approach described in (Rahman 2016), one could quantitate the layer thicknesses and the individual particle size as well as the distribution of the particles’ sizes on a wafer. The whole point of using a spun-on film is to eliminate the solvent and thus solvent effects, if any, on the particle size.
That withstanding, one must ask the question—How would one measure the size and/or the size distribution of nanoparticles, dispersed in a solvent such as the aforementioned nanoslurries? What happens if the nanoparticles remain in solution as opposed to being spun in to a film on a substrate? Can the “terahertz multispectral reconstructive imaging” (TMRI) protocol Rahman et al. (2016a, b) be effective for resolving these issues?
This brief account focuses on important applications resolved by DDE-based TMRI in the area of nanometrology. More specifically, it describes a facile protocol for real time, non-invasive characterization and quantitation of nanoparticle size/size distribution that may be measured in a variety of solvent environments.
Image formation by the terahertz multispectral reconstructive technique
The use of terahertz multispectral reconstructive imaging (TMRI) and terahertz time-domain spectrometry for characterizing semiconductor wafers and nanomaterials has been described elsewhere (Rahman et al. 2016a, b; Rahman 2017; Tomalia 2012; Tomalia and Khanna 2016). Firstly, TMRI offers an important opportunity to define pixel size (or a voxel size in 3D) by a hardware and software combination, as opposed to being limited by the image sensor chip such as the charge-coupled device (CCD). A digital camera, for example, displays and records the processed signal of an object that is focused on a CCD by means of a lens. The output of the CCD is processed by a built-in processor which displays the image and saves it in a file. In contrast, the reconstructive route eliminates the focusing lens and the CCD. Instead, the object to be imaged is scanned along the three orthogonal axes; the reflected signal (or, equivalently, the transmitted signal) is recorded in a data file and then processed by a suitable algorithm. The procedure for 3D image formation is described extensively elsewhere (Rahman et al. 2016a; Rahman 2017).
Description of silica nanoslurry
Particle size (nm)
7631-86-9. Ref: https://www.nissanchem-usa.com/products/organosilicasol/
As exhibited in Fig. 1, the source generates > 200 mW of CW power that is suitable for long-term operation. The output is very stable after the warmup time of ~ 2 h (Fig. 1). The source is capable of running continuously for several days at a time. This power is split into two arms for the built-in terahertz time-domain spectrometer implementation. The arms are termed as the pump arm that generates the terahertz via DDE emitter and the probe arm that detects the terahertz after being transmitted through or reflected from the sample. Both the terahertz source (DDE emitter) and the terahertz sensor are made of the same electro-optic dendrimer. The terahertz detection is carried out by the so-called electro-optic sampling technique, also known as pump-probing (Rahman 2011) technique.
Finally, both of the arms are combined and coupled into a multimode fiber for delivery to the nanoscanner. The nanoscanner deploys an optical circuitry for steering the beam and scanning an object for imaging via terahertz multispectral reconstructive technique described in Rahman et al. (2016b). As outlined in Fig. 2, the nanoslurry sample of the present investigation is kept in a short path-length cuvette. First, the empty cuvette is imaged and then the sample in the cuvette (nanoslurry) is imaged.
Results and discussion
In this paper, we have provided some details of the terahertz reconstructive imaging technique for the determination of the size and distribution of nanoparticles in suspension. Here, a high power, continuous wave terahertz source based on the Rahman-Tomalia effect (aka dendrimer dipole excitation) is utilized for designing a terahertz nanoscanning spectrometer and 3D imager that is used for all imaging experiments. In particular, a suspension of silica nanoparticles in isopropyl alcohol was investigated. Presence of nanoclusters was imaged along with their distribution in the suspension. It was also found qualitatively that the volume fraction of solvent is significantly higher than that of the silica nanoparticles; an observation consistent with the composition of the nanoslurry of the present investigation. Although the manufacturer’s specification indicates a nanoparticle distribution of 10 to 15 nm, most individual particles were actually found to be within a 11- to 12-nm range. However, a typical nanocluster size was determined to be 17.5 nm, thus indicating the presence of nanoparticles less than 10 nm.
Compliance with ethical standards
This research has been conducted at Applied Research and Photonics, Harrisburg, PA by the initiatives of Dr. Anis Rahman and in consultation with Dr. Donald Tomalia.
Conflict of interest
The authors declare that they have no conflict of interest.
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- Rahman A (2016) Simplest method of measuring nanoparticles’ size and size distribution on the surface and on sub-surface layers, Working Paper, December, pp. 1–5, DOI: https://doi.org/10.13140/RG.2.2.15414.50240. https://www.researchgate.net/publication/311542538_Simplest_method_of_measuring_nanoparticles%27_size_and_size_distribution_on_the_surface_and_on_sub-surface_layers
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