High Resolution Nanostructure

Table of Contents

Introduction
Demonstration of qmin Values for Silica Powder Characterization
Comparison of Scatterless Slit 2.0 with Standard Germanium Pinhole
Demonstration of High Signal-to-Noise Ratio
Conclusions

Introduction

Studies of nanostructures such as nanoparticle powders and suspensions aim to establish their characteristic lengths, such as interparticle diameter or distance. In SAXS study, these parameters are defined by fitting mathematical models to the collected data. Hence, accurate data is necessary to make useful interpretations.

The capability of the Xeuss 2.0 SAXS/WAXS system to expose the nanostructure of a sample is related to its ability to realize low detected wave vector (qmin) values, without sacrificing angular resolution.

Demonstration of qmin Values for Silica Powder Characterization

SAXS measurements, using a variety of slit apertures, have been performed on a SiO2 powder (Ø 150 nm) to establish the capability of the Xeuss 2.0 SAXS/WAXS system to realize a low qmin value combined with a high angular resolution.

The capability to acquire very low q values is shown by attaining scattering curves using very high resolution (VHR) and high resolution (HR) settings. This is illustrated in Figure 2. The beam is very clean at the beamstop edge, because of the Scatterless 2.0 technology. By shifting the beamstop as illustrated in the 2D pattern, ultimate qmin values down to 0.025 nm-1 are realized.

2D SAXS pattern of SiO2 powder with beamstop off-centered.

Figure 1. 2D SAXS pattern of SiO2 powder with beamstop off-centered.

Scattering curves from SiO2 powder.

Figure 2. Scattering curves from SiO2 powder. Influence of the collimation setting on the data quality. Exposure time = 600s.

Switching between various slit settings is achieved by a single instruction in the acquisition software, without any need for additional intervention. The same user-friendliness makes it possible to control the beam stop position.

Here, it was possible to view the particle signature up to about six oscillations, and related local minima are highlighted using the VHR setting. One can therefore choose between higher peak definitions with the VHR setting to enhance data analysis, while samples screening can be done using the HR setting.

At a sample-to-detector distance equal to 2.5 m, more than 14 experimental data points (no over sampling) between two local maxima of the scattering curve are presented, as illustrated in Figure 3. The related pixel resolution is equal to Δq = 0.003 nm-1. Hence, the system can examine particles up to 250 nm and beyond.

Inset in low q_region of the scattering curve for VHR setting.

Figure 3. Inset in low q_region of the scattering curve for VHR setting.

Comparison of Scatterless Slit 2.0 with Standard Germanium Pinhole

Measurement set-up.

Figure 4. Measurement set-up.

At BESSY II, tests were conducted at the PTB four-crystal monochromator beamline, in Berlin, Germany, using 8 keV radiation. The tested apertures were aligned to the main beam to enable a maximum photon flux. Sequential comparative measurements were performed between the commercially available scatterfree germanium pinhole and Scatterless slits 2.0.

The beam size was methodically varied with the S1 upstream slits, as depicted in Figure 4 for assessing the impact of photon flux on cleaning capabilities. In order to obtain a good comparison, the size of Scatterless slits 2.0 was fixed at 0.9 mm to get the same downstream photon flux as with the commercially available Ge-pinhole of 1 mm.

Figure 5 illustrates the comparative measurements (Data courtesy of Dr Michael Krumrey at PTB Berlin, with a four-crystal monochromator Beamline and 8 keV radiation). These 2D patterns reveal that the Scatterless slits 2.0 performance is more than that of commercially available Ge-pinhole, irrespective of the beam opening value.

Figure 6 compares the single dimensional scattering curves acquired from 2D patterns. In the measured q_range [0.04-0.4] nm-1, scattering from the Scatterless slits 2.0 remains lesser than that of the scatterfree Ge-pinhole. At low q value (0.04 nm-1), there is a ratio up to 10 favoring the Scatterless 2.0 slits.

2D scattering patterns obtained with various upstream slits aperture (S1)

Figure 5. 2D scattering patterns obtained with various upstream slits aperture (S1). 60 s exposure time.

Scattering curves obtained with S1 = 0.8 mm

Figure 6. Scattering curves obtained with S1 = 0.8 mm. 60 s exposure time.

Demonstration of High Signal-to-Noise Ratio

There is a requirement for a high signal-to-noise ratio (I/σ) at every q value so as to examine weak scattering systems such as diluted proteins or surfactant solutions. Merging Scatterless slits 2.0 technology with a low-noise camera in new generation Xeuss 2.0 SAXS/WAXS system guarantees high data quality collection and accurate analysis.

As illustrated in Figure 7, “empty camera”, 10 minute measurement reveals the ultra-low background level of the Xeuss 2.0. Figure 8 illustrates the following data in absolute intensity scale (mm-1):

  • Xeuss 2.0 typical beam profile as established on the detector
  • A typical scattering curve of a protein solution (10 mg/ml subF) not subtracted from buffer
  • Level of intensity of a typical polymer
  • Level of intensity of water

Scattering curves obtained with S1 = 0.8 mm

Figure 7. Zoom on the beamstop region at lowest qm No sample data. Exposure time 10 min.

Rebuilt 1D scattering curves from Xeuss camera beam profile and typical samples.

Figure 8. Rebuilt 1D scattering curves from Xeuss camera beam profile and typical samples.

The beam profile measurement reveals the ability of the Xeuss 2.0 to establish a clean beam. A clean beam suggests that there is an extremely low level of parasitic scattering propagated in the q-space meaning that the background signal stays low even at small q-values.

The noise level is noticed to be lower than the water scattering intensity allowing reliable water measurement. The diluted protein’s scattering curve demonstrates a higher scattering than the noise signal, particularly at high q values, showing the ability to perform correct buffer subtraction on the collected data.

Furthermore, considering the incident beam of the Xeuss 2.0 SAXS/WAXS system, the signal-to-noise ratio is higher than 4x109. This high level of signal-to-noise ratio capacity is an outcome from the integration of the Low Noise technologies created by Xenocs in the Xeuss 2.0 SAXS/WAXS system, such as the Scatterless slits 2.0. Using a Dectris detector helps in taking total advantage of these features.

Conclusions

In applications such as colloids and polymer materials, superior quality SAXS investigation of much larger nanostructures with characteristic lengths of 500 nm is of interest. It requires a qmin value down to 0.01 nm-1 or below. Superior quality data analysis can only be done with an associated nominal resolution equal to Δq = 0.001 nm-1. This guarantees data with a sufficient number of data points and enables accurate model fitting procedure. The USAXS version of the Xeuss 2.0 SAXS/WAXS system offers such features.

Application fields like characterization of diluted systems, i.e., BioSAXS, require high signal-to-noise ratio (I/σ) at every q values, to guarantee enhanced buffer subtraction for accurate data analysis. Likewise, examination of large nanostructures with weak scattering power includes high resolution collimation which effects useful flux on sample. For such difficult applications, high I/σ at low qmin is very vital, and increasing such factor offers a direct enhancement in data quality.

Using low scattering slits contributes to enhancing the signal-to-noise ratio. The Scatterless slits 2.0 are completely integrated in the new generation Xeuss 2.0 SAXS/WAXS system. Further to enhancing performance, the Scatterless 2.0 slits enable one to alter the SAXS/WAXS system resolution by automatic recall of slits settings via a single software push button action without additional intervention.

The Xeuss 2.0 SAXS/WAXS system also exhibits a high signal-to-noise ratio that enables SAXS measurements of proteins. Measurements performed on protein sub-F reveal that consistent data is attained compared to synchrotron results and enables the protein structure resolution.

Xenocs

This information has been sourced, reviewed and adapted from materials provided by Xenocs.

For more information on this source, please visit Xenocs.

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