1、Designation: E2859 11Standard Guide forSize Measurement of Nanoparticles Using Atomic ForceMicroscopy1This standard is issued under the fixed designation E2859; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revis
2、ion. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 The purpose of this document is to provide guidance onthe quantitative application of atomic force microscopy (AFM)to determine
3、the size of nanoparticles2deposited in dry form onflat substrates using height (z-displacement) measurement.Unlike electron microscopy, which provides a two-dimensionalprojection or a two-dimensional image of a sample, AFMprovides a three-dimensional surface profile. While the lateraldimensions are
4、influenced by the shape of the probe, displace-ment measurements can provide the height of nanoparticleswith a high degree of accuracy and precision. If the particlesare assumed to be spherical, the height measurement corre-sponds to the diameter of the particle. In this guide, proceduresare describ
5、ed for dispersing gold nanoparticles on varioussurfaces such that they are suitable for imaging and heightmeasurement via intermittent contact mode AFM. Genericprocedures for AFM calibration and operation to make suchmeasurements are then discussed. Finally, procedures for dataanalysis and reporting
6、 are addressed. The nanoparticles used toexemplify these procedures are National Institute of Standardsand Technology (NIST) reference materials containing citrate-stabilized negatively charged gold nanoparticles in an aqueoussolution.1.2 The values stated in SI units are to be regarded asstandard.
7、No other units of measurement are included in thisstandard.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bi
8、lity of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3E1617 Practice for Reporting Particle Size CharacterizationDataE2382 Guide to Scanner and Tip Related Artifacts inScanning Tunneling Microscopy and Atomic Force Mi-croscopyE2456 Terminology Relating to Nanotechnol
9、ogyE2530 Practice for Calibrating the Z-Magnification of anAtomic Force Microscope at Subnanometer DisplacementLevels Using Si(111) Monatomic StepsE2587 Practice for Use of Control Charts in StatisticalProcess Control2.2 ISO Standards:4ISO 18115-2 Surface ChemicalAnalysis - Vocabulary - Part2: Terms
10、 Used in Scanning-Probe MicroscopyISO/IEC Guide 98-3:2008 Uncertainty of measurementPart 3: Guide to the Expression of Uncertainty in Mea-surement (GUM:1995)3. Terminology3.1 For definitions pertaining to nanotechnology terms,refer to Terminology E2456.3.2 For definitions pertaining to terms associa
11、ted withscanning-probe microscopy, including AFM, refer toISO 18115-2.3.3 Definitions of Terms Specific to This Standard:3.3.1 agglomerate, nin nanotechnology, an assembly ofparticles held together by relatively weak forces (for example,Van der Waals or capillary), that may break apart into smallerp
12、articles upon processing, for example. E24563.3.1.1 DiscussionUsing imaging based techniques, suchas AFM, it is generally difficult to differentiate between1This guide is under the jurisdiction of ASTM Committee E56 on Nanotech-nology and is the direct responsibility of Subcommittee E56.02 on Charac
13、terization:Physical, Chemical, and Toxicological Properties.Current edition approved Dec. 1, 2011. Published January 2012. DOI: 10.1520/E2859-11.2Having two or three dimensions in the size scale from approximately 1 nm to100 nm as in accordance with Terminology E2456; this definition does not consid
14、erfunctionality, which may impact regulatory aspects of nanotechnology, but whichare beyond the scope of this guide.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to
15、the standards Document Summary page onthe ASTM website.4Available from International Organization for Standardization (ISO), 1, ch. dela Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http:/www.iso.org.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 1942
16、8-2959, United States.agglomerates formed during the deposition process (that is,artifacts) and agglomerates or aggregates that pre-exist in thetest sample.3.3.2 aggregate, nin nanotechnology, a discrete assem-blage of particles in which the various individual componentsare not easily broken apart,
17、such as in the case of primaryparticles that are strongly bonded together (for example, fused,sintered, or metallically bonded particles). E24563.3.2.1 DiscussionUsing imaging based techniques, suchas AFM, it is generally difficult to differentiate betweenaggregates and agglomerates.3.4 Acronyms:3.4
18、.1 AFMatomic force microscopy3.4.2 APDMES3-aminopropyldimethylethoxysilane3.4.3 DIdeionized3.4.4 HEPAhigh efficiency particulate air3.4.5 NISTNational Institute of Standards and Technol-ogy3.4.6 PLLpoly-L-lysine3.4.7 RMreference material4. Summary of Practice4.1 This standard guide outlines the proc
19、edures for samplepreparation and the determination of nanoparticle size usingatomic force microscopy (AFM). An AFM utilizes a cantileverwith a sharp probe to scan a specimen surface. The cantileverbeam is attached at one end to a piezoelectric displacementactuator controlled by the AFM. At the other
20、 end of thecantilever is the probe tip that interacts with the surface. Atclose proximity to the surface, the probe experiences a force(attractive or repulsive) due to surface interactions, whichimposes a bending moment on the cantilever. In response tothis moment, the cantilever deflects, and this
21、deflection ismeasured using a laser beam that is reflected from a mirroredsurface on the back side of the cantilever onto a splitphotodiode. A schematic diagram of the system is shown inFig. 1. The cantilever deflection is measured by the differentialoutput (difference in responses of the upper and
22、lower sections)of the split photodiode. The deflections are very small relativeto the cantilever thickness and length. Thus, the probe displace-ment is linearly related to the deflection. The cantilever istypically silicon or silicon nitride with a tip radius of curvatureon the order of nanometers.
23、More detailed and comprehensiveinformation on the AFM technique and its applications can befound in the published literature (1, 2).54.2 Based on the nature of the probe-surface interaction(attractive or repulsive), an AFM can be selected to operate invarious modes, namely contact mode, intermittent
24、 contactmode, or non-contact mode. In contact mode, the interactionbetween the tip and surface is repulsive, and the tip literallycontacts the surface. At the opposite extreme, the tip interactswith the surface via long-range surface force interactions; thisis called non-contact mode. In intermitten
25、t contact mode (alsoreferred to as tapping mode), the cantilever is oscillated closeto its resonance frequency perpendicular to the specimensurface, at separations closer to the sample than in non-contactmode. As the oscillating probe is brought into proximity withthe surface, the probe-surface inte
26、ractions vary from longrange attraction to weak repulsion and, as a consequence, theamplitude (and phase) of the cantilever oscillation varies.During a typical imposed 100 nm amplitude oscillation, for ashort duration of time, the tip extends into the repulsive regionclose to the surface, intermitte
27、ntly touching the surface andthereby reducing the amplitude. Intermittent contact mode hasthe advantage of being able to image soft surfaces or particlesweakly adhered to a surface and is hence preferred fornanoparticle size measurements.4.3 A microscope feedback mechanism can be employed tomaintain
28、 a user definedAFM set point amplitude, in the case ofintermittent contact mode. When such feedback is operational,constant vibration amplitude can be maintained by displacingthe built-in end of the cantilever up and down by means of thepiezo-actuator.5The boldface numbers in parentheses refer to a
29、list of references at the end ofthis standard.FIG. 1 Schematic Illustration of AFM Measurement PrincipleE2859 112NOTE 1Operation of an AFM with feedback off enables the interac-tions to be measured and this is known as force spectroscopy.This displacement directly corresponds to the height of thesam
30、ple. A topographic image of the surface can be generatedby rastering the probe over the specimen surface and recordingthe displacement of the piezo-actuator as a function of position.Although the lateral dimensions are influenced by the shape ofthe probe (see Guide E2382 for guidance on tip relateda
31、rtifacts), the height measurements can provide the height ofnanoparticles deposited onto a substrate with a high degree ofaccuracy and precision. If the particles are assumed to bespherical, the height measurement corresponds to the diameteror “size” of the particle.4.4 Procedures for dispersing nan
32、oparticles on various sur-faces such that they are suitable for imaging and heightmeasurement via intermittent contact mode AFM are firstdescribed. The nanoparticles used to exemplify these proce-dures were National Institute of Standards and Technology(NIST) gold nanoparticle reference materials, R
33、M 8011 (nomi-nally 10 nm), RM 8012 (nominally 30 nm), and RM 8013(nominally 60 nm), all of which contained citrate-stabilizednegatively charged gold nanoparticles in an aqueous solution.4.5 Generic procedures for AFM calibration and operationto perform size measurements in intermittent contact mode
34、arediscussed, and procedures for data analysis and reporting areoutlined.5. Significance and Use5.1 As AFM measurement technology has matured andproliferated, the technique has been widely adopted by thenanotechnology research and development community to theextent that it is now considered an indis
35、pensible tool forvisualizing and quantifying structures on the nanoscale.Whether used as a stand-alone method or to complement otherdimensional measurement methods, AFM is now a firmlyestablished component of the nanoparticle measurement toolbox. International standards for AFM-based determination o
36、fnanoparticle size are nonexistent as of the drafting of thisguide. Therefore, this standard aims to provide practical andmetrological guidance for the application of AFM to measurethe size of substrate-supported nanoparticles based on maxi-mum displacement as the probe is rastered across the partic
37、lesurface to create a line profile.6. Reagents6.1 Certain chemicals and materials may be necessary inorder to perform one or more of the steps discussed in thisguide, but the specific reagents used are at the discretion of thetester and may depend on which specific alternative proceduresare chosen o
38、r relevant for a particular application.6.2 Adhesive tape, if needed to cleave mica substrates.6.3 Atomically flat gold (111) on mica, if needed as asubstrate material.6.4 Colloidal gold, citrate-stabilized in aqueous solution,ifneeded to test or validate sample preparation and measurementprocedures
39、.6.5 Deionized water, filtered to 0.1 m, as needed for samplepreparation or to rinse substrates.6.6 Ethanol, reagent or chromatographic grade, as neededto rinse substrates.6.7 HCl, concentrated (37 %), if needed to clean silicon (Si)substrates.6.8 H2O2, 30 % solution, if needed to clean Si substrate
40、s.6.9 Inert compressed gas source (for example, nitrogen,argon, or air), filtered to remove particles.6.10 Mica disc, if needed as a substrate material.6.11 Poly-l-lysine, solution (0.1 %), if needed for prepara-tion of functionalized substrates.6.12 Single crystal Si wafers, diced to appropriate si
41、ze,ifneeded as a substrate material.7. Apparatus7.1 Atomic Force Microscope, capable of makingz-displacement measurements at sub-nanoscale dimensions.7.2 Bath Ultrasonicator, as needed to clean substrates.7.3 Microcentrifuge (“Microfuge”), as needed for samplepreparation.7.4 RF Plasma Cleaner with O
42、2, as needed to clean Sisubstrates.8. Procedure8.1 Nanoparticle DepositionFor AFM measurements,nanoparticle samples must be deposited on flat surfaces. Theroughness of the surface should be much less than the nominalsize of the nanoparticles (preferably less than 5 %) in order toprovide a consistent
43、 baseline for height measurements. High-quality mica, atomically flat gold (111) (deposited on mica), orsingle crystal silicon can all be used as substrates to minimizethe effect of surface roughness on nanoparticle measurements.Example procedures are provided for depositing nanoparticleson these th
44、ree substrates. The sample deposition proceduresoutlined below were developed for use with negatively chargedcitrate-stabilized gold nanoparticles suspended in an aqueoussolution at a mass concentration nominally 50 g/g (as exem-plified by NIST RMs 8011, 8012, and 8013). The proceduresshould work wi
45、th other nanoparticles that carry a negativesurface charge or zeta potential, including, but not limited to,commercially available citrate-stabilized colloidal gold. Assuggested below, these procedures can also be applied topositively charged or neutral nanoparticles with some modifi-cation. Each pr
46、ocedure may require optimization by the user inorder to obtain satisfactory deposition density and to minimizeartifacts such as agglomerate formation on the substrate orbuild-up of organic films resulting from additives that might bepresent in the solution phase.NOTE 2Substrate preparation and sampl
47、e deposition should be con-ducted in a manner that minimizes the potential for contamination andartifacts. For instance, to the extent possible, these operations should beconducted in a HEPA filtered clean bench or work area. Similarly,prepared samples should be stored in a manner that maintains the
48、irintegrity and precludes contamination.8.1.1 Mica SubstrateMica is a layered mineral that can bereadily cleaved along alkali-rich basal planes to form clean,atomically flat surfaces extending over large areas. To preparethe substrate, a mica disc must be cleaved to produce a cleansurface. Place the
49、 disc on a clean, lint-free cloth or directly onan AFM puck. Press a piece of adhesive tape against theE2859 113surface of the disc and then smoothly remove the tape from themica. The top layer of the mica should appear on the tape.Continue to cleave the mica until a full layer is removed andthe exposed surface is visually smooth. Typically, this stepneeds to be repeated several times, and requires visual inspec-tion of the cleaved surface.8.1.1.1 After cleaving, the mica disc is ready to be activatedso as to promote adhesion between the substrate and the goldnano
