{"id":{"repo_id":"uiuc","oai_identifier":"oai:www.ideals.illinois.edu:2142/102912"},"canonical_url":"https://search.dev.ndltd.org/etd/uiuc/oai:www.ideals.illinois.edu:2142/102912","repository":{"repo_id":"uiuc","name":"University of Illinois - Urbana-Champaign","base_url":"https://www.ideals.illinois.edu/oai-pmh"},"display":{"title":"Spectroscopic surface scattering of confined acoustic phonons in silicon nanostructures","abstract":"The specularity of phonons at rough crystal surfaces is a fundamental aspect of phonon transport in nanostructures. It directly impacts engineering problems such as heat conduction in nanostructures and dissipation in nanomechanical resonators. At room temperature, the available guidance from theory is limited to fully diffuse transport under the condition of small surface roughness. Recent experiments in thermal transport suggest that there may exist large gaps in understanding phonon interactions with rough surfaces, especially when the roughness dimensions are comparable to phonon wavelengths. To consider such refinements, this thesis focuses on spectroscopic measurements of specularity in silicon nanostructures with well-characterized surface morphologies. We employ a femtosecond laser pump-probe setup to excite and detect confined acoustic phonons (∼ 18 - 200 GHz) in freely- suspended silicon membranes and nanowires. Surface scattering dominates intrinsic Akhiezer damping at frequencies > 60 GHz, thereby enabling us to probe phonon-boundary interactions over wavelengths ∼ 42 - 140 nm. To quantitatively understand the dependence of boundary scattering on RMS roughness and correlation length, we obtained detailed statistics of the surfaces using HRTEM and AFM imaging. For silicon membranes, we find that both Ziman and perturbation approach for roughness scattering successfully explain the nearly specular reflection of ∼ 0.1 THz phonons from surface with ∼ 1-nm scale roughness. The measured phonon specularities for silicon nanowires, however, are significantly lower in comparison to membranes for the frequency range ν ∼ 18−100 GHz. The reduction in specularity is caused by additional scattering from multiple surfaces introduced in a nanowire. Using a remarkably simple normalization scheme, we show that the scattering from multiple surfaces can be effectively decoupled. The magnitudes of the (normalized) phonon lifetimes are in good quantitative agreement with the predictions of Ziman approach but does not perfectly explain the frequency dependence. The τ ∼ ν^−1.7 dependence observed in our experiments is suggestive of weak phonon localization which cannot be understood within the existing framework of single scattering (or equivalently first Born approximation). This work helps to advance the fundamental understanding of phonon scattering at the surfaces of nanostructures.","abstract_html":"The specularity of phonons at rough crystal surfaces is a fundamental aspect of phonon transport in nanostructures. It directly impacts engineering problems such as heat conduction in nanostructures and dissipation in nanomechanical resonators. At room temperature, the available guidance from theory is limited to fully diffuse transport under the condition of small surface roughness. Recent experiments in thermal transport suggest that there may exist large gaps in understanding phonon interactions with rough surfaces, especially when the roughness dimensions are comparable to phonon wavelengths. To consider such refinements, this thesis focuses on spectroscopic measurements of specularity in silicon nanostructures with well-characterized surface morphologies. We employ a femtosecond laser pump-probe setup to excite and detect confined acoustic phonons (∼ 18 - 200 GHz) in freely- suspended silicon membranes and nanowires. Surface scattering dominates intrinsic Akhiezer damping at frequencies > 60 GHz, thereby enabling us to probe phonon-boundary interactions over wavelengths ∼ 42 - 140 nm. To quantitatively understand the dependence of boundary scattering on RMS roughness and correlation length, we obtained detailed statistics of the surfaces using HRTEM and AFM imaging. For silicon membranes, we find that both Ziman and perturbation approach for roughness scattering successfully explain the nearly specular reflection of ∼ 0.1 THz phonons from surface with ∼ 1-nm scale roughness. The measured phonon specularities for silicon nanowires, however, are significantly lower in comparison to membranes for the frequency range ν ∼ 18−100 GHz. The reduction in specularity is caused by additional scattering from multiple surfaces introduced in a nanowire. Using a remarkably simple normalization scheme, we show that the scattering from multiple surfaces can be effectively decoupled. The magnitudes of the (normalized) phonon lifetimes are in good quantitative agreement with the predictions of Ziman approach but does not perfectly explain the frequency dependence. The τ ∼ ν^−1.7 dependence observed in our experiments is suggestive of weak phonon localization which cannot be understood within the existing framework of single scattering (or equivalently first Born approximation). This work helps to advance the fundamental understanding of phonon scattering at the surfaces of nanostructures.","abstract_has_math":false,"creators":["Gelda, Dhruv"],"institution":"University of Illinois at Urbana-Champaign","degree_name":"Ph.D.","degree_level":"Dissertation","degree_discipline":"Mechanical Engineering","degree_department":null,"school":null,"contributors":["Sinha, Sanjiv","Bahl, Gaurav","Li, Xiuling","Matlack, Kathryn"],"advisors":[],"committee_chairs":[],"committee_members":[],"year":2019,"date_issued":"2019-02-08T18:39:46Z","date_published":"2019-02-08T18:39:46Z","updated_at":"2026-07-22T22:24:42Z","subjects":["boundary scattering, acoustic phonons, silicon nanostructures"],"languages":["en"],"rights":["Copyright 2018 Dhruv Gelda"],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"http://hdl.handle.net/2142/102912","outbound_label":"Handle","outbound_source":"dc:identifier"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Sinha, Sanjiv","Bahl, Gaurav","Li, Xiuling","Matlack, Kathryn"]},{"key":"dc:creator","label":"Author","values":["Gelda, Dhruv"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date","label":"Dc Date","values":["2019-02-08T18:39:46Z","2021-02-09T10:15:21Z","2018-11-15","2018-12"]},{"key":"dc:type","label":"Dc Type","values":["text"]},{"key":"thesis:degree_discipline","label":"Discipline","values":["Mechanical Engineering"]},{"key":"thesis:degree_level","label":"Degree Level","values":["Dissertation"]},{"key":"thesis:degree_name","label":"Degree Name","values":["Ph.D."]},{"key":"thesis:institution_name","label":"Thesis Institution Name","values":["University of Illinois at Urbana-Champaign"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["boundary scattering, acoustic phonons, silicon nanostructures"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["en"]},{"key":"dc:rights","label":"Dc Rights","values":["Copyright 2018 Dhruv Gelda"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier","label":"Identifier","values":["http://hdl.handle.net/2142/102912"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["The specularity of phonons at rough crystal surfaces is a fundamental aspect of phonon transport in nanostructures. It directly impacts engineering problems such as heat conduction in nanostructures and dissipation in nanomechanical resonators. At room temperature, the available guidance from theory is limited to fully diffuse transport under the condition of small surface roughness. Recent experiments in thermal transport suggest that there may exist large gaps in understanding phonon interactions with rough surfaces, especially when the roughness dimensions are comparable to phonon wavelengths. To consider such refinements, this thesis focuses on spectroscopic measurements of specularity in silicon nanostructures with well-characterized surface morphologies. We employ a femtosecond laser pump-probe setup to excite and detect confined acoustic phonons (∼ 18 - 200 GHz) in freely- suspended silicon membranes and nanowires. Surface scattering dominates intrinsic Akhiezer damping at frequencies > 60 GHz, thereby enabling us to probe phonon-boundary interactions over wavelengths ∼ 42 - 140 nm. To quantitatively understand the dependence of boundary scattering on RMS roughness and correlation length, we obtained detailed statistics of the surfaces using HRTEM and AFM imaging. For silicon membranes, we find that both Ziman and perturbation approach for roughness scattering successfully explain the nearly specular reflection of ∼ 0.1 THz phonons from surface with ∼ 1-nm scale roughness. The measured phonon specularities for silicon nanowires, however, are significantly lower in comparison to membranes for the frequency range ν ∼ 18−100 GHz. The reduction in specularity is caused by additional scattering from multiple surfaces introduced in a nanowire. Using a remarkably simple normalization scheme, we show that the scattering from multiple surfaces can be effectively decoupled. The magnitudes of the (normalized) phonon lifetimes are in good quantitative agreement with the predictions of Ziman approach but does not perfectly explain the frequency dependence. The τ ∼ ν^−1.7 dependence observed in our experiments is suggestive of weak phonon localization which cannot be understood within the existing framework of single scattering (or equivalently first Born approximation). This work helps to advance the fundamental understanding of phonon scattering at the surfaces of nanostructures.","Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2020-12-01","The student, Dhruv Gelda, accepted the attached license on 2018-11-15 at 12:58.","The student, Dhruv Gelda, submitted this Dissertation for approval on 2018-11-15 at 13:10.","This Dissertation was approved for publication on 2018-11-15 at 16:13.","DSpace SAF Submission Ingestion Package generated from Vireo submission #13089 on 2019-02-08 at 11:39:00","Made available in DSpace on 2019-02-08T18:39:46Z (GMT). No. of bitstreams: 3 GELDA-DISSERTATION-2018.pdf: 14036561 bytes, checksum: f0e895e64a14f5a62224056be47d9e8c (MD5) LICENSE.txt: 4208 bytes, checksum: 3aa75b8301b3c609bfe55bc94dada74a (MD5) PROQUEST_LICENSE.txt: 4554 bytes, checksum: ac0e75b5720a80dcd4d81e55e1768bd5 (MD5) Previous issue date: 2018-11-15","Embargo set by: Seth Robbins for item 109938 Lift date: 2021-02-08T18:40:00Z Reason: Author requested closed access (OA after 2yrs) in Vireo ETD system","Embargo set by: Seth Robbins for item 109938 Lift date: 2021-02-08T18:42:23Z Reason: Author requested closed access (OA after 2yrs) in Vireo ETD system","Embargo set by: Seth Robbins for item 109938 Lift date: 2021-02-08T18:43:54Z Reason: Author requested closed access (OA after 2yrs) in Vireo ETD system","Embargo set by: Seth Robbins for item 109938 Lift date: 2021-02-08T18:44:50Z Reason: Author requested closed access (OA after 2yrs) in Vireo ETD system","Limited Restriction Lifted for Item 109938 on 2021-02-09T10:15:21Z."]},{"key":"dc:format","label":"Dc Format","values":["application/pdf"]},{"key":"dc:title","label":"Title","values":["Spectroscopic surface scattering of confined acoustic phonons in silicon nanostructures"]}]}],"canonical_facts":{"dc:contributor":["Sinha, Sanjiv","Bahl, Gaurav","Li, Xiuling","Matlack, Kathryn"],"dc:creator":["Gelda, Dhruv"],"dc:date":["2019-02-08T18:39:46Z","2021-02-09T10:15:21Z","2018-11-15","2018-12"],"dc:description":["The specularity of phonons at rough crystal surfaces is a fundamental aspect of phonon transport in nanostructures. It directly impacts engineering problems such as heat conduction in nanostructures and dissipation in nanomechanical resonators. At room temperature, the available guidance from theory is limited to fully diffuse transport under the condition of small surface roughness. Recent experiments in thermal transport suggest that there may exist large gaps in understanding phonon interactions with rough surfaces, especially when the roughness dimensions are comparable to phonon wavelengths. To consider such refinements, this thesis focuses on spectroscopic measurements of specularity in silicon nanostructures with well-characterized surface morphologies. We employ a femtosecond laser pump-probe setup to excite and detect confined acoustic phonons (∼ 18 - 200 GHz) in freely- suspended silicon membranes and nanowires. Surface scattering dominates intrinsic Akhiezer damping at frequencies > 60 GHz, thereby enabling us to probe phonon-boundary interactions over wavelengths ∼ 42 - 140 nm. To quantitatively understand the dependence of boundary scattering on RMS roughness and correlation length, we obtained detailed statistics of the surfaces using HRTEM and AFM imaging. For silicon membranes, we find that both Ziman and perturbation approach for roughness scattering successfully explain the nearly specular reflection of ∼ 0.1 THz phonons from surface with ∼ 1-nm scale roughness. The measured phonon specularities for silicon nanowires, however, are significantly lower in comparison to membranes for the frequency range ν ∼ 18−100 GHz. The reduction in specularity is caused by additional scattering from multiple surfaces introduced in a nanowire. Using a remarkably simple normalization scheme, we show that the scattering from multiple surfaces can be effectively decoupled. The magnitudes of the (normalized) phonon lifetimes are in good quantitative agreement with the predictions of Ziman approach but does not perfectly explain the frequency dependence. The τ ∼ ν^−1.7 dependence observed in our experiments is suggestive of weak phonon localization which cannot be understood within the existing framework of single scattering (or equivalently first Born approximation). This work helps to advance the fundamental understanding of phonon scattering at the surfaces of nanostructures.","Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2020-12-01","The student, Dhruv Gelda, accepted the attached license on 2018-11-15 at 12:58.","The student, Dhruv Gelda, submitted this Dissertation for approval on 2018-11-15 at 13:10.","This Dissertation was approved for publication on 2018-11-15 at 16:13.","DSpace SAF Submission Ingestion Package generated from Vireo submission #13089 on 2019-02-08 at 11:39:00","Made available in DSpace on 2019-02-08T18:39:46Z (GMT). No. of bitstreams: 3 GELDA-DISSERTATION-2018.pdf: 14036561 bytes, checksum: f0e895e64a14f5a62224056be47d9e8c (MD5) LICENSE.txt: 4208 bytes, checksum: 3aa75b8301b3c609bfe55bc94dada74a (MD5) PROQUEST_LICENSE.txt: 4554 bytes, checksum: ac0e75b5720a80dcd4d81e55e1768bd5 (MD5) Previous issue date: 2018-11-15","Embargo set by: Seth Robbins for item 109938 Lift date: 2021-02-08T18:40:00Z Reason: Author requested closed access (OA after 2yrs) in Vireo ETD system","Embargo set by: Seth Robbins for item 109938 Lift date: 2021-02-08T18:42:23Z Reason: Author requested closed access (OA after 2yrs) in Vireo ETD system","Embargo set by: Seth Robbins for item 109938 Lift date: 2021-02-08T18:43:54Z Reason: Author requested closed access (OA after 2yrs) in Vireo ETD system","Embargo set by: Seth Robbins for item 109938 Lift date: 2021-02-08T18:44:50Z Reason: Author requested closed access (OA after 2yrs) in Vireo ETD system","Limited Restriction Lifted for Item 109938 on 2021-02-09T10:15:21Z."],"dc:format":["application/pdf"],"dc:identifier":["http://hdl.handle.net/2142/102912"],"dc:language":["en"],"dc:rights":["Copyright 2018 Dhruv Gelda"],"dc:subject":["boundary scattering, acoustic phonons, silicon nanostructures"],"dc:title":["Spectroscopic surface scattering of confined acoustic phonons in silicon nanostructures"],"dc:type":["text"],"thesis:degree_discipline":["Mechanical Engineering"],"thesis:degree_level":["Dissertation"],"thesis:degree_name":["Ph.D."],"thesis:institution_name":["University of Illinois at Urbana-Champaign"]},"updated_at":"2026-07-22T22:24:42Z"}