Search with Scope using PowerShell CSOM on SharePoint

Add-Type -Path "C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"
Add-Type -Path "C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Search.dll"

$siteURL = ""
$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($siteURL)

#Not required for on premise site - Start
$userId = ""
$pwd = Read-Host -Prompt "Enter password" -AsSecureString
$creds = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($userId, $pwd)
$ctx.credentials = $creds
#Not required for on premise site - End

try{
    $searchquery = New-Object Microsoft.SharePoint.Client.Search.Query.KeywordQuery($ctx)
    $searchquery.QueryText = "nakkeeran"

    #Result source GUID's for specific set of results can be identified
    #using the Search Service Application's Result Source Section.
    #The result source can be people, document, video, etc.
    $searchquery.SourceId = "00000000-0000-0000-000000000000" #Document

    $searchExec = New-Object Microsoft.SharePoint.Client.Search.Query.SearchExecutor($ctx)
    $searchResults = $searchExec.ExecuteQuery($searchquery)
    $ctx.ExecuteQuery()
}
catch{
    write-host "$($_.Exception.Message)" -foregroundcolor red
}